Methods of forming semiconductor fiber and optoelectronic fiber

Abstract

Various embodiments may relate to a method of forming a semiconductor fiber. The method may include determining, via a computer, a suitable combination of a fiber core material and a cladding material. The method may also include providing the fiber core material into an inner cavity of a tube, the tube including the cladding material, to form a composite structure. The method may further include subjecting the composite structure to a thermal drawing process at a draw temperature higher than a melting point of the fiber core material to form the semiconductor fiber including a core including the fiber core material, and a cladding around the core, the cladding including the cladding material.

Description

METHODS OF FORMING SEMICONDUCTOR FIBER AND OPTOELECTRONICFIBERCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore application No. 10202400275V filed January 31, 2024, the contents of it being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] Various embodiments of this disclosure may relate to a method of forming a semiconductor fiber. Various embodiments of this disclosure may relate to a method of forming an optoelectronic fiber.BACKGROUND

[0003] Recent breakthroughs in fiber technology enable the assembly of insulating, semiconducting, and conducting materials with intimate interfaces and microscopic features into a single fiber with specific geometries. Woven into pristine fabrics, functional fabrics are formed to deliver diverse functionalities over a large area, for example, serving as sensors, actuators, energy generation and storage, and healthcare apparatus. Optoelectronic fibers are particularly promising to engage various applications in imaging, communication, environment sensing, and health monitoring. As semiconductors are the critical components that primarily govern the device performance, the selection, control, and engineering of semiconductors inside fibers provide the key pathway to address the challenges of achieving high-performance optoelectronic fibers. Glassy semiconductors, such as chalcogenide glasses, are commonly used in thermally drawn fibers due to their low processing temperatures and controllable fluidic behaviors. However, compared with crystalline semiconductors that are widely used in electronics such as silicon (Si) and germanium (Ge), the inevitable high densities of electronic defects in glassy semiconductors often lead to poor electrical properties of the fabricated fibers. Thus, the use of crystalline semiconductors is more favorable to fundamentally boost the further development of optoelectronic fibers. To obtain continuous long crystalline semiconductor fibers, various crystal growth techniques were developed, such as Czochralski, Bridgman-Stockbarger, float zone, and micro-pulling-down methods. However, the growthrates and fabrication lengths are commonly limited to a few centimeters per hour and tens of centimeters, respectively.SUMMARY

[0004] Various embodiments may relate to a method of forming a semiconductor fiber. The method may include determining, via a computer, a suitable combination of a fiber core material and a cladding material. The method may also include providing the fiber core material into an inner cavity of a tube, the tube including the cladding material, to form a composite structure. The method may further include subjecting the composite structure to a thermal drawing process at a draw temperature higher than a melting point of the fiber core material to form the semiconductor fiber including a core including the fiber core material, and a cladding around the core, the cladding including the cladding material. The suitable combination of the fiber core material and the cladding material may be determined based on a closeness of a softening point of the cladding material and the melting point of the fiber core material, a closeness of a coefficient of thermal expansion (CTE) of the cladding material and a coefficient of thermal expansion (CTE) of the fiber core material, and a total growth factor of the composite structure at the draw temperature and based on provided parameters of the core of the semiconductor fiber.

[0005] Various embodiments may relate to a method of forming an optoelectronic fiber. The method may include forming one or more semiconductor fibers according to any method as described herein. The method may also include removing the cladding of each of the one or more semiconductor fibers to expose the one or more cores. The method may further include subjecting the one or more cores with one or more metal wires, a first polymeric material and a second polymeric material to a convergence fiber drawing process to form the optoelectronic fiber.BRIEF DESCRIPTION OF THE DRAWINGS

[0006] In the drawings, reference characters generally refer to the same parts throughout the different views. The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In thefollowing description, various embodiments of the invention are described with reference to the following drawings.FIG. 1 shows a general illustration of a method of forming a semiconductor fiber according to various embodiments.FIG. 2 shows a general illustration of a method of forming an optoelectronic fiber according to various embodiments.FIG. 3A shows a schematic of the molten core method.FIG. 3B shows schematics and corresponding microscopy images of various fiber core geometries that correspond with different degrees of capillary instability.FIG. 3C shows schematics and corresponding microscopy images of various fiber core geometries (intact and cracked cores) that correspond with different stress levels formed during fabrication.FIG. 3D shows a schematic illustrating hundreds of continuous semiconductor fibers fabricated from one thermal drawing process, as well as a magnified microscopy image of the thermally drawn fibers according to various embodiments.FIG. 3E shows a schematic illustrating the convergence fiber drawing process according to various embodiments.FIG. 3F shows a schematic of functional fabrics enabled by the resulting optoelectronic fibers according to various embodiments.FIG. G shows a schematic illustrating the applications for optoelectronic fibers according to various embodiments.FIG. 4A shows a schematic illustrating the solidification of a small column of the liquid semiconductor core over a short time of At according to various embodiments.FIG. 4B shows a schematic illustrating the thermal mismatch at the cooling stage according to various embodiments.FIG. 5A shows a microscopy image of cracks observed in an as-drawn fiber with germanium (Ge) core.FIG. 5B shows a microscopy image of germanium (Ge) segments after etching away the cladding.FIG. 5C shows a microscopy image showing a lateral cross-section of the fiber with cracks.FIG. 5D shows a scanning electron microscopy (SEM) image showing the transverse cross- sectional of the fiber with cracks.FIG. 6A shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified silicon (Si)Zsilica (SiCh) fiber according to various embodiments.FIG. 6B shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified germanium (Ge)Zsilica (Si(h) fiber according to various embodiments.FIG. 6C shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified germanium (Ge) / aluminosilicate glass (ASG) fiber according to various embodiments.FIG. 6D shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the silicon (Si) core / silica cladding according to various embodiments.FIG. 6E shows a plot of stress component G (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the germanium (Ge) core / silica cladding according to various embodiments.FIG. 6F shows a plot of stress component G (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the germanium (Ge) core / aluminosilicate glass (ASG) cladding according to various embodiments.FIG. 7A shows a plot of stress component G (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the silicon (Si) core of a Si / silica fiber according to various embodiments.FIG. 7B shows a plot of stress component G (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the silica cladding of a Si / silica fiber according to various embodiments.FIG. 7C shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the germanium (Ge) core of a Ge / silica fiber according to various embodiments.FIG. 7D shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the silica cladding of a Ge / silica fiber according to various embodiments.FIG. 7E shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the germanium (Ge) core of a Ge / aluminosilicate glass (ASG) fiber according to various embodiments.FIG. 7F shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the aluminosilicate glass (ASG) cladding of a Ge / ASG fiber according to various embodiments.FIG. 8A shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) comparing Raman spectra of a raw silicon (Si) rod, a silicon (Si)Zsilica fiber, and a released silicon (Si) fiber according to various embodiments.FIG. 8B shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating an enlarged image of FIG. 8A around the peak position according to various embodiments.FIG. 8C shows a Raman mapping of the cross-section of the silicon (Si)Zsilica fiber according to various embodiments.FIG. 8D shows a plot of peak center (per centimeter or cm'1) as a function of position (in micrometer or pm) illustrating a line scan across the circular cross-section passing through the center of the silicon (Si)Zsilica fiber according to various embodiments.FIG. 8E shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) comparing Raman spectra of a raw germanium (Ge) rod, a germanium (Ge)Zsilica fiber, a germanium (Ge)Z aluminosilicate glass (ASG) fiber, and a released Ge fiber according to various embodiments.FIG. 8F shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating an enlarged image of FIG. 8E around the peak position according to various embodiments.FIG. 8G shows a Raman mapping of the cross-section of the germanium (Ge)Z aluminosilicate glass (ASG) fiber according to various embodiments.FIG. 8H shows a plot of peak center (per centimeter or cm'1) as a function of position (in micrometer or pm) illustrating a line scan across the circular cross-section passing through the center of the germanium (Ge)Z aluminosilicate glass (ASG) fiber according to various embodiments.FIG. 9 A shows schematics illustrating a breakup in the core of a germanium (Ge) / borosilicate glass (BSG) system: (above) Ge spheres are observed in the neck-down region, and (below) perturbated Ge core in the resulting fiber.FIG. 9B shows a plot of radius (in centimeters or cm) as a function of axial (Z) position (in centimeters or cm) illustrating an iterative calculation of the neck profile according to various embodiments.FIG. 9C shows microscopy images of (above) a neck profile of a preform without the semiconductor core (outline represents calculated neck profile); and (below) a neck profile of a preform with the semiconductor core (outline represents calculated neck profile) according to various embodiments.FIG. 9D shows a plot of core radius (in micrometers or pm) as a function of draw temperature (in degrees Celsius or °C) illustrating a contour map of the growth factor for a germanium (Ge) / borosilicate glass (BSG) fiber according to various embodiments.FIG. 9E shows a plot of core radius (in micrometers or pm) as a function of draw temperature (in degrees Celsius or °C) illustrating a contour map of the growth factor for a germanium (Ge) / aluminosilicate glass (ASG) fiber according to various embodiments.FIG. 10A shows a schematic of a silicon (Si) / silica fiber under bending according to various embodimentsFIG. 10B shows a microscopy image of the fiber in FIG. 10A according to various embodiments.FIG. IOC shows a dark-field microscopy image of the polished transverse cross-section of the silicon (Si) / silica fiber according to various embodiments.FIG. 10D is a dark-field microscopy image of the polished lateral cross-section of the silicon (Si) / silica fiber according to various embodiments.FIG. 10E shows a scanning electron microscopy (SEM) image of a line scan that was applied to the transverse cross-section of the silicon (Si) / silica fiber according to various embodiments. FIG. 10F shows a plot of concentration (in atomic percent or at. %) as a function of relative distance (in micrometers) illustrating energy dispersive X-ray (EDX) results of a line scan that was applied to the transverse cross-section of the silicon (Si) / silica fiber according to various embodiments.FIG. 10G shows a plot of intensity (in arbitrary units or a.u.) as a function of angle 20 (in degrees) illustrating the X-ray diffraction patterns of the silicon (Si) / silica fiber according to various embodiments.FIG. 10H shows a high-resolution transmission electron microscopy (HRTEM) image of the silicon (Si) / silica fiber according to various embodiments, with the inset showing a selected area electron diffraction (SAED) image of the silicon (Si) / silica fiber according to various embodiments that confirms the [Fd3ml] space group.FIG. 101 shows a schematic of the germanium / aluminosilicate glass (Ge / ASG) fiber under bending according to various embodiments.FIG. 10J shows a microscopy image of the fiber in FIG. 101 according to various embodiments. FIG. 1 OK shows a dark-field microscopy image of the polished transverse cross-section of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments.FIG. 10L shows a dark-field microscopy image of the polished lateral cross-section of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments.FIG. 10M shows a scanning electron microscopy (SEM) image of a line scan that was applied to the transverse cross-section of a germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments.FIG. I ON shows a plot of concentration (in atomic percent or at. %) as a function of relative distance (in micrometers) illustrating energy dispersive X-ray (EDX) results of a line scan that was applied to the transverse cross-section of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments.FIG. 100 shows a plot of intensity (in arbitrary units or a.u.) as a function of angle 20 (in degrees) illustrating the X-ray diffraction patterns of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments.FIG. 10P shows a high resolution transmission electron microscopy (HRTEM) image of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments, with the inset showing a selected area electron diffraction (SAED) image of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments that confirms the [Fd3ml] space group.FIG. 10Q shows a microscopy image of a standalone silicon (Si) fiber of 55 pm diameter after the removal of glass cladding according to various embodiments.FIG. 10R is a schematic showing that the standalone silicon (Si) fiber according to various embodiments can be bent with a bending radius of 4 mm.FIG. IOS shows (left) a schematic illustrating that the standalone silicon (Si) fiber according to various embodiments can easily hold the weight of a 70 g computer mouse; and (right) a plot illustrating that the Si fiber according to various embodiments experiences a tensile strength of 338.39 ± 143.83 MPa when holding the computer mouse.FIG. 10T is a schematic showing that an 80 cm-long standalone silicon (Si) fiber according to various embodiments can be safely wrapped on a bobbin due to its mechanical strength.FIG. 10U shows a microscopy image of a standalone germanium (Ge) fiber of 55 pm diameter after the removal of glass cladding according to various embodiments.FIG. 10V is a schematic showing that the standalone germanium (Ge) fiber according to various embodiments can be bent with a bending radius of 2.5 mm.FIG. 10W shows (left) a schematic illustrating that the standalone germanium (Ge) fiber according to various embodiments can easily hold the weight of a 86 g mouse; and (right) a plot illustrating that the Ge fiber according to various embodiments experiences a tensile strength of 505.85 ± 105.41 MPa when holding the computer mouse.FIG. 1 OX is a schematic showing that an 80 cm-long standalone germanium (Ge) fiber according to various embodiments can be safely wrapped on a bobbin due to its mechanical strength.FIG. 11 A is a microscopy image of the cross-sectional side view of a single-core optoelectronic fiber according to various embodiments, where the core semiconductor (silicon (Si) or germanium (Ge) is connected to each copper electrode through a layer of carbon-loaded polycarbonate (CPC) according to various embodiments.FIG. 1 IB is a microscopy image of the transverse cross-section of the single-core optoelectronic fiber in FIG. 1 1 A according to various embodimentsFIG. 11C shows a schematic illustrating that approximately 50 meters of optoelectronic fiber according to various embodiments may be collected in a single draw.FIG. 1 ID shows stimulated electric field distribution along the length of the optoelectronic fiber according to various embodiments, illustrating that the electric field along the fiber length is uniform.FIG. 1 IE shows a plot of current (in micro- Amperes or pA) as a function of time (in seconds or s) illustrating that the resulting optoelectronic fiber according to various embodiments shows a pseudo-omnidirectional response that maintains sensitivity in different directions.FIG. 1 1F shows a plot of current (in micro-Amperes or pA) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a silicon (Si) optoelectronic fiber according to various embodiments under different illumination conditions.FIG. 11G shows a plot of current (in micro-Amperes or pA) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a germanium (Ge) optoelectronic fiber according to various embodiments under different illumination conditions.FIG. 12A is a microscopy image of the cross-sectional side view of a dual-core p-n junction fiber according to various embodiments.FIG. 12B is a microscopy image of the transverse cross-section of the dual-core p-n junction fiber in FIG. 12A according to various embodiments.FIG. 12C shows a plot of current (in amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristic of the p-n junction fiber according to various embodiments.FIG. 13 A shows a schematic illustrating an overall performance evaluation of the resulting silicon (Si) and germanium (Ge) optoelectronic fibers according to various embodiments.FIG. 13B shows a plot of current (in arbitrary units or a.u.) as a function of time (in seconds or s) illustrating the photo response of the optoelectronic fibers according to various embodiments before, during and after bending in cyclic testing, as well before and after wash testing.FIG. 13C shows a microscopy image of twisted optoelectronic fibers according to various embodiments, which may still maintain functionality even with a turn density of three turns per mm.FIG. 13D shows (above) a plot of pressure (in Mega-Pascals or MPa) as a function of time (in seconds or s) illustrating the pressure-time history of the fibers according to various embodiments during a compression test where a maximum compressive stress of 30 MPa was applied to the fibers according to various embodiments; and (below) a plot of current (in micro- Amperes or pA) as a function of time (in seconds or s) illustrating the photoresponse of the fibers according to various embodiments during the compression test.FIG. 13E shows a schematic illustrating a functional fabric including fibers according to various embodiments being washed in a washing machine.FIG. 13F shows a thermal image indicating that the temperatures of the optoelectronic fibers according to various embodiments remained unchanged after five hours of continuous operation.FIG. 14A shows a schematic illustrating the weaving process to form a functional fabric according to various embodiments.FIG. 14B shows a schematic illustrating a functional beanie including a functional fabric according to various embodiments.FIG. I4C shows (bottom left) a schematic illustrating a functional sweater including the functional fabric according to various embodiments which makes use of a light fidelity (Li-Fi) based indoor communication system to receive information of a particular building; (top left) a schematic of the functional sweater with the fiber grid according to various embodiments, and (right) a block diagram illustrating the receiving of data via the functional sweater according to various embodiments.FIG. 14D shows (top left) a schematic illustrating a smartwatch with a watchband including the functional fabric for measuring heart pulses according to various embodiments; (bottom left) a magnified schematic of the smartwatch according to various embodiments; and (right) plots of current (in arbitrary units or a.u.) against time (s) comparing results of a commercial sensor and a fiber according to various embodiments.FIG. 14E shows (left) a schematic of a fiber receiver array including the optoelectronic fibers as part of an underwater visible light communication system according to various embodiments; (bottom right) a schematic of a mini-submarine with optoelectronic fibers attached to form the array and coupled to a printed circuit board (PCB) according to various embodiments; and (top right) a top view schematic of the mini-submarine with optoelectronic fibers according to various embodiments.FIG. 15A shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor silicon (Si) core of a silicon (Si) / silica fiber for different fiber radii (n) while keeping the same core radius (n=50 pm) according to various embodiments.FIG. 15B shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor silicon (Si) core of a silicon (Si) / silica fiber for different values of A / according to various embodiments.FIG. 15C shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge)Zsilica fiber for different fiber radii (n) while keeping the same core radius (ri=50 pm) according to various embodiments.FIG. 15D shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge) / silica fiber for different values of At according to various embodiments.FIG. 15E shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge) / aluminosilicate glass (ASG) fiber for different fiber radii (rz) while keeping the same core radius (n=50 pm) according to various embodiments.FIG. 15F shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge) / aluminosilicate glass (ASG) fiber for different values of J / according to various embodiments.DESCRIPTION

[0007] The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, logical, and electrical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.

[0008] Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and / orcombinations and / or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0009] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0010] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance, e.g., within 10% of the specified value.

[0011] As used herein, the term “and / or” includes any and all combinations of one or more of the associated listed items.

[0012] By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.

[0013] By “consisting of’ it is meant including, and limited to, whatever follows the phrase “consisting of’. Thus, the phrase “consisting of’ indicates that the listed elements are required or mandatory, and that no other elements may be present.

[0014] Embodiments described in the context of one of the fibers are analogously valid for the other fibers. Similarly, embodiments described in the context of a method are analogously valid for a fiber, and vice versa.

[0015] FIG. 1 shows a general illustration of a method of forming a semiconductor fiber according to various embodiments. The method may include, in 102, determining, via a computer, a suitable combination of a fiber core material and a cladding material. The method may also include, in 104, providing the fiber core material into an inner cavity of a tube, the tube including the cladding material, to form a composite structure. The method may further include, in 106, subjecting the composite structure to a thermal drawing process at a draw temperature higher than a melting point of the fiber core material to form the semiconductor fiber including a core including the fiber core material, and a cladding around the core, the cladding including the cladding material. The suitable combination of the fiber core material and the cladding material may be determined based on a closeness of a softening point (or annealing point) of the cladding material and the melting point of the fiber core material, a closeness of a coefficient of thermal expansion (CTE) of the cladding material and a coefficient of thermal expansion (CTE) of the fiber core material, and a total growth factor of thecomposite structure at the draw temperature and based on provided parameters of the core of the semiconductor fiber.

[0016] In other words, various embodiments may relate to a method of forming a semiconductor fiber by first determining a suitable combination of a fiber core material and a cladding material based on three factors, i.e., a difference between a softening point (or annealing point) of the cladding material and a melting point of the core material, a difference between a closeness of a coefficient of thermal expansion (CTE) of the cladding material and a coefficient of thermal expansion (CTE) of the core material, as well as a total growth factor of the composite structure at the draw temperature calculated or determined based on provided parameters of the core of the semiconductor fiber. The method may further include providing the fiber core material into a tube of the cladding material to form a composite structure. Thermal drawing may be carried out by having the draw temperature higher than the melting point of the fiber core material to melt the fiber core material to form the semiconductor fiber including a core including the fiber core material and a cladding around the core, the cladding including the cladding material.

[0017] In various embodiments, the provided parameters of the core of the semiconductor fiber may include a diameter of the core and / or a geometry of the core. In other words, the total growth factor of the composite structure at the draw temperature may be based on the diameter of the core and / or the geometry of the core (i.e., shape formed by the perimeter or the circumference of a transverse cross-section of the core of the semiconductor fiber).

[0018] In various embodiments, the core material may undergo a melting process, followed by a solidification stage and a cooling stage during the thermal drawing process.

[0019] In various embodiments, the term “computer” may refer to any processor, controller, electronic device or circuit arrangement that is suitable for determining the suitable combination of the fiber core material and the cladding material. In various embodiments, the computer may be any suitable general-purpose computer, tablet or smartphone. In various embodiments, the term “computer” may refer to a network of processors, controllers, electronic devices and / or circuit arrangements that collectively is suitable for determining the suitable combination of the fiber core material and the cladding material. Different parts of the determination or computation may be carried out by different devices of the network. The computer may include or be configured to process a non-transitory computer-readable medium or media having computer-executable instructions for execution by the processor(s),controller(s), electronic device(s) and / or circuit arrangement(s) to cause the processor(s), controller(s), electronic device(s) and / or circuit arrangement(s) to perform the step of determining the suitable combination of the fiber core material and the cladding material.

[0020] Tn various embodiments, the suitable combination of the fiber core material and the cladding material may be determined by requiring the total growth factor to be below 1.

[0021] In various embodiments, the suitable combination of the fiber core material and the cladding material may be determined by requiring the softening point of the cladding material to be 50 °C to 200 °C higher than the melting point of the fiber core material.

[0022] In various embodiments, the suitable combination of the fiber core material and the cladding material may be determined by requiring that a difference between the coefficient of thermal expansion (CTE) of the cladding material and the coefficient of thermal expansion (CTE) of the core material to be equal to or below 2.8 10'6K '' (i.e., 2.8 x 10'6K-1).

[0023] Various embodiments may relate to a method of forming a semiconductor fiber including providing a fiber core material into an inner cavity of a tube, the tube including a cladding material, to form a composite structure, and subjecting the composite structure to a thermal drawing process at a draw temperature higher than a melting point of the fiber core material to form the semiconductor fiber, such that the total growth factor of the composite structure at the draw temperature is below 1 . Also, the softening point of the cladding material may be a value selected from a range from 50 °C to 200 °C higher than the melting point of the fiber core material. Further, a difference between a coefficient of thermal expansion (CTE) of the cladding material and a coefficient of thermal expansion (CTE) of the core material may be a value selected from a range from 0 K to 2.8 - 10'6K-1.

[0024] In various embodiments, the total growth factor may be determined based on a temperature distribution, a viscosity distribution and a velocity distribution along an axial position of a necking region of the composite structure at the draw temperature and provided parameters of the core of the semiconductor fiber.

[0025] In various embodiments, the total growth factor may be determined via iterative calculations.

[0026] In various embodiments, the fiber core material may be or may include any suitable material, for instance, any one selected from a group consisting of silicon (Si), germanium (Ge), silicon-germanium (SiGe), gallium arsenide (GaAs), gallium antimonide (GaSb), gallium nitride (GaN), and silver sulfide (Ag S).

[0027] In various embodiments, the fiber core material may be or may include germanium, and the cladding material may be or may include aluminosilicate glass (ASG) In various other embodiments, the fiber core material may be or may include silicon, and the cladding material may be or may include silica (SiCh)

[0028] In various embodiments, during the thermal drawing process, the composite structure including the tube of cladding material and the fiber core material including within an inner cavity of the tube, may be drawn into a thinner semiconductor fiber including the cladding and the core. An outer diameter of the tube may be greater than an outer diameter of the cladding. The region of the composite structure adjoining the formed semiconductor fiber may be referred to as a “necking region” or “neck-down” region. In the necking region or neck- down region, the outer diameter of the composite structure may be become shorter with decreasing distance from the formed semiconductor fiber.

[0029] In various embodiments, the core of the semiconductor fiber formed may be devoid of cracks or may contain very few cracks. Various embodiments may relate to a long semiconductor fiber, e.g., having a length of 100 meters or above in which the core is devoid of cracks or contains very few cracks. Various embodiments may relate to a method capable of forming 10 meters or above of the semiconductor fiber per minute in a single thermal drawing process.

[0030] Various embodiments may relate to a semiconductor fiber as described herein or formed by a method as described herein.

[0031] FIG. 2 shows a general illustration of a method of forming an optoelectronic fiber according to various embodiments. The method may include, in 202, forming one or more semiconductor fibers according to any method as described herein. The method may also include, in 204, removing the cladding of each of the one or more semiconductor fibers to expose the one or more cores. The method may further include, in 206, subjecting the one or more cores with one or more metal wires, a first polymeric material and a second polymeric material to a convergence fiber drawing process to form the optoelectronic fiber.

[0032] In other words, the method may include removing the claddings from one or more semiconductor fibers as described herein to obtain one or more cores, and using convergence drawing fiber drawing to form an optoelectronic fiber from the one or more cores, one or more metal wires, a first polymeric material and a second polymeric material.

[0033] In various embodiments, the cladding of each of the one or more semiconductor fibers may be removed by chemical etching.

[0034] In various embodiments, the optoelectronic fiber may include a polymeric intermediate layer including the first polymeric material surrounding the one or more cores. The optoelectronic fiber may also include one or more metal wires substantially parallel to the one or more cores (i.e., the length(s) of the one or more metal wires are substantially parallel to the length(s) of the one or more cores) such that the polymeric intermediate layer is between the one or more cores and the one or more metal wires. The phase “one or more metal wires substantially parallel to the one or more cores” may mean that the one or more metal wires are parallel to the one or more cores, or may deviate by an angle of e.g., less than ± 5°, e.g., ± 3°, e.g., ±1 °. In various embodiments, the one or more metal wires may be at any suitable angles to the one or more cores. Generally speaking, a smaller angle of deviation from the case in which the one or more metal wires are perfectly parallel to the one or more cores may improve a performance of the optoelectronic fiber. The optoelectronic fiber may also include a polymeric outer layer including the second polymeric material around the one or more metal wires.

[0035] In various embodiments, the first polymeric material may be or may include any suitable conductive thermoplastic material. For instance, the suitable conductive thermoplastic material may be or may include carbon-loaded thermoplastic material, liquid metal-loaded thermoplastic material or silver particles-loaded thermoplastic material.

[0036] In various embodiments, the suitable conductive thermoplastic material may be or may include the carbon-loaded thermoplastic material, the carbon-loaded thermoplastic material being or including carbon-loaded polycarbonate (CPC) or carbon-loaded polyethylene.

[0037] In various embodiments, the second polymeric material may be or may include any suitable thermoplastic material. For instance, the suitable thermoplastic material may be or may include polycarbonate (PC), polyethylene, polystyrene, cyclic olefin copolymer, or polyetherimide.

[0038] In various embodiments, the one or more metal wires may be or may include any suitable metal For instance, the one or more metal wires may be or may include copper, tungsten, titanium, nickel, aluminum, steel, zinc, brass, silver, or gold.

[0039] In various embodiments, the one or more cores may be two cores. The one or more cores may include first p-doped core and a second n-doped core.

[0040] Various embodiments may relate to an optoelectronic fiber as described herein or formed by a method as described herein. Various embodiments may relate to a functional fabric or article (e.g., beanie, sweater smartwatch, watch strap, mini-submarine etc.) including one or more optoelectronic fibers. Various embodiments may include a functional fabric or article including one or more optoelectronic fibers and a printed circuit board (PCB) coupled to the one or more optoelectronic fibers.

[0041] To achieve high-yield production of semiconductor fibers at extended lengths, the molten core method was demonstrated. FIG. 3 A shows a schematic of the molten core method. Using this method, the semiconductor core material is melted into a fluid flow that is confined by the glass cladding and thermally drawn into fibers. Over hundreds of meters of semiconductor fiber can thus be produced at the speed of a few tens of meters per minute in a single drawing process. Compared with crystal growth techniques, this high yield method processes materials far from the equilibrium. The interface between glass cladding and semiconductor core contributes significantly to the complex stress development in the core, leading to either perturbed or fractured fiber, which fundamentally restricts the production of large-scale optoelectronic fibers with high performance. FIG. 3B shows schematics and corresponding microscopy images of various fiber core geometries that correspond with different degrees of capillary instability. FIG. 3C shows schematics and corresponding microscopy images of various fiber core geometries (intact and cracked cores) that correspond with different stress levels formed during fabrication. Various embodiments may relate to a mechanical design to achieve ultra-long, continuous, perturbation-free, and fracture-free semiconductor fibers.

[0042] Various embodiments may relate to a system for understanding the morphology and stress development at the three stages of the fiber formation by the molten core method: the viscous flow, the core crystallization, and the subsequent cooling stage. Various embodiments may make use of the above to achieve continuous semiconductor fibers. FIG. 3D shows a schematic illustrating hundreds of continuous semiconductor fibers fabricated from one thermal drawing process, as well as a magnified microscopy image of the thermally drawn fibers according to various embodiments. Further, optoelectronic fibers made of silicon (Si) and germanium (Ge) single-core structures and dual-core structures may be formed viaconvergence drawing. FIG. 3E shows a schematic illustrating the convergence fiber drawing process according to various embodiments Taking advantage of the fiber form factor, the resulting optoelectronic fibers may cover a long sensing length and may conform to curved surfaces where traditional rigid and discrete photodetectors may not be applied. The mechanical robustness of the resulting optoelectronic fibers may allow them to be woven into large-scale fabrics, constructing functional fabrics, while maintaining favorable features like conformability, machine washability, and permeability. FIG. 3F shows a schematic of functional fabrics enabled by the resulting optoelectronic fibers according to various embodiments. Such optoelectronic fibers may offer comparable performance to the commercial planar-type photodetectors and may enable the full integration with power supply, signal collection and wireless data transmission modules on a small printed circuit board (PCB), achieving diverse applications. FIG. 3G shows a schematic illustrating the applications for optoelectronic fibers according to various embodiments.

[0043] To trace the stress formation in the molten core method, two stages may be identified: the core solidification stage and the subsequent cooling stage. FIG. 4A shows a schematic illustrating the solidification of a small column of the liquid semiconductor core over a short time of At according to various embodiments. The position of the liquid-solid interface may remain the same in the steady-state thermal drawing process. FIG. 4B shows a schematic illustrating the thermal mismatch at the cooling stage according to various embodiments. AEQ and AEQ may represent the increases in the thermal strains of the core and the cladding, respectively, under the temperature change A7'.

[0044] At the core solidification stage, semiconductor cores are confined by the glass cladding, and the anomalous expansion during the crystallization of silicon (Si) and germanium (Ge) cores may contribute to the stress formation. Then, the core and cladding may experience different thermal strains during annealing. Thus, the combination of anomalous expansion and thermal mismatch may lead to high stresses in semiconductor cores, bringing up the necessity of rational mechanical design to avoid cracks and fractures, especially when the removal of glass cladding is required.

[0045] During the core solidification stage, since drawing a long fiber at constant velocity is a steady-state process, the liquid-solid interface of the semiconductor core may remain at the same position after a short period of At. The newly solidified portion of the core in At with a length of I — vAt, where v is the drawing speed, has experienced a crystallization-inducedvolume expansion from the corresponding liquid column of the length l0before the solidification (FIG. 4A). The volumes of liquid Si and Ge may be increased by 8 % ~ 9 % upon crystallization, equivalent to an axial expansion coefficient a0= - of the core confined within 'o the fiber cladding, which can be determined by finite element (FE) simulations. For Ge / silica fiber, cracks were observed in the core, while acceptable core quality was achieved in Si / silica fiber.

[0046] FIG. 5A shows a microscopy image of cracks observed in an as-drawn fiber with germanium (Ge) core. FIG. 5B shows a microscopy image of germanium (Ge) segments after etching away the cladding. FIG. 5C is a microscopy image showing a lateral cross-section of the fiber with cracks. FIG. 5D is a scanning electron microscopy (SEM) image showing the transverse cross-sectional of the fiber with cracks.

[0047] The different viscoelastic responses of the cladding near the liquid-solid interface during the solidification of the semiconductor core are responsible for these distinct results of Ge / silica and Si / silica fibers. Glass claddings typically exhibit viscoelastic behavior that relieves stresses at high temperatures, which is closely related to the viscosity. The viscosity of vitreous silica varies widely across more than 5 orders of magnitude between the melting points of Si (T^ = 1410°C)and GeAs a result, silica glass cladding may provide distinct lateral restrictions to the Si and Ge cores For Si / silica fiber, the internal stresses in the cores are relaxed shortly (< 5 s) after the solidification, due to the highly viscous behavior of the silica glass cladding at the melting point of the Si core FIG 6A shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified silicon (Si) / silica (SiO2) fiber according to various embodiments. FIG. 6B shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified germanium (Ge) / silica (SiOz) fiber according to various embodiments. In contrast, stress relaxation in the Ge / silica fiber needs a significantly longer time (> 1000 s), and a high compressive stress remains in Ge during the drawing process, due to the considerable gap between T-^eand the annealing point of silicon (T$lllca— 1200°C). Thus, the persistent high compression brings a substantial risk of forming transverse cracks in the confined Ge core in silica glass cladding as shown in FIGS. 5A - D. To achieve rational mechanical design, aluminosilicate glass (ASG) may be introduced as the cladding material for drawing Ge core fiber. Compared to silicon glass, ASG has a much lower annealing point (TaSG— 795°C), andexhibits a more viscous behavior at T^ , which contributes to the rapid relaxation (< 1 s) of the internal stress in the Ge core. FIG. 6C shows a plot of stress (in Giga-Pascals or GPa) as a function of time (in seconds or s) illustrating evolution of the maximum principal stress in the solidified germanium (Ge) / aluminosilicate glass (ASG) fiber according to various embodiments. Selecting ASG as cladding material may reduce the likelihood of the Ge core from cracking at the core solidification stage.

[0048] When entering the cooling stage, as the crystalized core is drawn away from the furnace, the mismatch in thermal expansion rates of the core and cladding induces additional stresses in both fiber core and cladding as shown in FIG. 4B. A mechanical model was developed to study the stress distributions due to such thermal mismatch (see Supplementary Note 1 below). Since silica glass has a much smaller coefficient of thermal expansion (CTE) than that of Si and Ge semiconductor cores, FIGS. 6D and 6E show that the high axial tensions are generated in the cores after the cooling FIG. 6D shows a plot of stress component G (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the silicon (Si) core / silica cladding according to various embodiments. FIG. 6E shows a plot of stress component o (in MegaPascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the germanium (Ge) core / silica cladding according to various embodiments. The tensile stresses can promote the propagation of the existing cracks initiated at the solidification stage or even induce new cracks in the Ge core, while the Si core survives, benefiting from its higher strength than Ge. However, in the Ge / ASG fiber, the cladding has a CTE (see Supplementary Table 1 below) close to that of Ge, resulting in small thermal mismatch and axial tension in the core, as shown in FIG. 6F. FIG. 6F shows a plot of stress component c (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) illustrating the radial distribution of the stress components in the fiber with the germanium (Ge) core / aluminosilicate glass (ASG) cladding according to various embodiments Accordingly, a continuous Ge core without cracks can be obtained

[0049] FIGS. 7A and 7B show the comparison of theoretical and FE results on the stress distributions in Si core and silica cladding. FIG. 7A shows a plot of stress component a (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the silicon (Si) core of a Si / silica fiber according to various embodiments. FIG. 7B shows a plot of stress component o (in Mega-Pascals or MPa) as afunction of radius (in micrometers or jam) comparing theoretical and finite element (FE) results of the silica cladding of a Si / silica fiber according to various embodiments FIGS. 7C and 7D show the comparison of theoretical and FE results on the stress distributions in Ge core and silica cladding FIG. 7C shows a plot of stress component o (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the germanium (Ge) core of a Ge / silica fiber according to various embodiments. FIG. 7D shows a plot of stress component c> (in Mega-Pascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the silica cladding of a Ge / silica fiber according to various embodiments. FIGS. 7E and 7F show the comparison of theoretical and FE results on the stress distributions in the Ge core and ASG cladding after being cooled to the ambient temperature. FIG 7E shows a plot of stress component G (in MegaPascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the germanium (Ge) core of a Ge / aluminosilicate glass (ASG) fiber according to various embodiments. FIG. 7F shows a plot of stress component c (in MegaPascals or MPa) as a function of radius (in micrometers or pm) comparing theoretical and finite element (FE) results of the aluminosilicate glass (ASG) cladding of a Ge / ASG fiber according to various embodiments.

[0050] FIGS. 7A - 7F show that the predicted stress distributions by the model are in excellent agreement with FE simulation results. In contrast to only being able to obtain Ge fragments from the Ge / silica fiber (upon removal of the silica cladding), a continuous standalone Ge fiber can be obtained via the removal of the cladding from the Ge / ASG fiber (upon removal of the ASG cladding).

[0051] The stress analyses indicate that both the volume expansion of semiconductors at the solidification stage and the thermal expansion mismatch at the cooling stage contribute to crack formation in the Ge core confined by a silica cladding Moreover, the effect of the drawing force on core cracking can be neglected, as it produces much smaller fiber tensions (10 - 20 MPa) than those induced by the solidification expansion and thermal mismatch. The overall stress level in the core can be measured through Raman spectroscopy as shown in FIGS. 8A - 8H. Note that the results cannot precisely reveal the pristine stress value due to the necessary polish for sample preparation, but they can serve as a reference for qualitative analysis.

[0052] FIG. 8A shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) comparing Raman spectra of a raw silicon (Si) rod, a silicon (Si) / silica fiber, and a released silicon (Si) fiber according to various embodiments. FIG. 8B shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating an enlarged image of FIG. 8 A around the peak position according to various embodiments. The curve relating to Si / silica fiber shows a blueshift, indicating the formation of tensile stress, while the curve relating to released Si fiber shows minor residual stress. FIG. 8C shows a Raman mapping of the cross-section of the silicon (Si)Zsilica fiber according to various embodiments. A uniform stress distribution is observed at the inner part, while a ring of tensile stress area around the interface can also be observed. FIG. 8D shows a plot of peak center (per centimeter or cm'1) as a function of position (in micrometer or pm) illustrating a line scan across the circular cross-section passing through the center of the silicon (Si)Zsilica fiber according to various embodiments. FIG. 8E shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) comparing Raman spectra of a raw germanium (Ge) rod, a germanium (Ge)Zsilica fiber, a germanium (Ge)Z aluminosilicate glass (ASG) fiber, and a released Ge fiber according to various embodiments. FIG. 8F shows a plot of intensity (in arbitrary units or a.u.) as a function of Raman shift (per centimeter or cm'1) illustrating an enlarged image of FIG. 8E around the peak position according to various embodiments. The curve relating to the GeZASG fiber shows a smaller redshift than the curve relating to the GeZsilica fiber, indicating minor compressive stress. FIG. 8G shows a Raman mapping of the cross-section of the germanium (Ge)Z aluminosilicate glass (ASG) according to various embodiments. Due to stress formed during cooling, the interface has a smaller redshift than the center area. Scratches caused by polishing are also observed. FIG. 8H shows a plot of peak center (per centimeter or cm'1) as a function of position (in micrometer or pm) illustrating a line scan across the circular cross-section passing through the center of the germanium (Ge)Z aluminosilicate glass (ASG) according to various embodiments.

[0053] The Si and Ge cores in silica cladding possess positive and negative residual stresses, respectively (see Supplementary Table 2 below), presumably resulting from the small relaxed compression at the solidification plus an axial tension induced by the thermal mismatch for the Si core, and from the huge compression due to the solidification expansion plus a high axial tension (yet smaller than the compression in magnitude) at the cooling stage for the Ge core. Also note that the as-drawn Ge core in silica cladding was broken into short segments, whichmay release a part of the internal stresses accumulated during the thermal drawing. On the other hand, the Ge core in ASG cladding shows a small residual stress because of the lower stress induced at both stages. However, precisely quantitative comparison between modelling (or simulation) and experimental results to determine the individual contribution of solidification expansion and thermal mismatch is currently challenging due to the extreme working conditions. Nevertheless, the modelling and FE simulation results may provide informative guidance for the fabrication of semiconductor fibers with continuous cores via the molten core method: to choose the cladding materials with (1) an annealing point close to the melting point of the core and (2) a CTE close to that of the core material.

[0054] FIG. 9A shows schematics illustrating a breakup in the core of a germanium (Ge) / borosilicate glass (BSG) system: (above) Ge spheres are observed in the neck-down region, and (below) perturbated Ge core in the resulting fiber. FIG. 9B shows a plot of radius (in centimeters or cm) as a function of axial (Z) position (in centimeters or cm) illustrating an iterative calculation of the neck profile according to various embodiments. FIG. 9C shows microscopy images of (above) a neck profile of a preform without the semiconductor core (outline represents calculated neck profile); and (below) a neck profile of a preform with the semiconductor core (outline represents calculated neck profile) according to various embodiments The preforms with and without the semiconductor cores are quenched to obtain the neck profiles.

[0055] Before the solidification of the semiconductor core, the whole fiber may remain in the combination of a cylindrical viscous fluid (core) in another viscous fluid (cladding), subject to the capillary instability. Capillary instability may be unfavored as it disturbs the continuity and consistency of the cylindrical fiber core at the geometry along both radial and axial directions as shown in FIGS. 3B and 9A. Thus, a criterion describing the growth of capillary instability related to the draw parameters may be required to study the phenomenon and optimize process parameters. It can be established by modifying a classical model built for two viscous cylindrical liquids with the consideration of a necking profile (see Supplementary Note 2 below), which can be calculated through an iterative method as shown in FIGS. 9B - C. Via this modelling, a total growth factor of capillary instability can be obtained to indicate the magnitude of the exponential growth of capillary instability in the core, thus serving as the criterion for the development of capillary instability in the resulting fibers. Complete breakup in the core due to capillary instability may be expected for a total growth factor far greater thanone, while the total growth factor with a value much smaller than one may indicate stability in the core.

[0056] In addition to silica glass, borosilicate glass (BSG) has also been reported to be used as a cladding material for drawing Ge core fibers The drawing temperature is commonly set to at least 1000 °C to make sure molten Ge is achieved within the dwell time of travelling in the furnace. However, BSG has a low softening point (825 °C) and maintains a comparatively low viscosity at a drawing temperature of 1000 °C, which brings a high capillary instability growth rate. As a result, perturbations can be observed in the Ge core, especially with slow drawing speeds or small core diameters. To ensure sufficient melting of the Ge core, it may not be possible to suppress the growth of perturbations by lowering the drawing temperature. To address this issue, a cladding material with a softening point slightly higher than the melting point of Ge is expected. ASG, with a softening point of 1005 °C, may satisfy this requirement and may be a suitable cladding material for hosting a Ge core, considering it also possesses a suitable annealing point and thermal expansion coefficient to suppress cracking, as discussed above. Comparisons of the total growth factor of drawing Ge core fibers with BSG and ASG are shown in the contour maps (FIGS 9D - E). FIG. 9D shows a plot of core radius (in micrometers or pm) as a function of draw temperature (in degrees Celsius or °C) illustrating a contour map of the growth factor for a germanium (Ge) / borosilicate glass (BSG) fiber according to various embodiments. FIG. 9E shows a plot of core radius (in micrometers or pm) as a function of draw temperature (in degrees Celsius or °C) illustrating a contour map of the growth factor for a germanium (Ge) / aluminosilicate glass (ASG) fiber according to various embodiments. Regions have been indicated in FIGS. 9D - E to denote drawing conditions in which the semiconductor core is safe from the growth of capillary instability. The plot relating to Ge / ASG fiber in FIG. 9E has a much greater area denoting that the semiconductor core is safe from the growth of capillary instability compared to the plot relating to Ge / BSG fiber in FIG. 9D. The results may indicate that a broader safe window of drawing temperature and core diameter is allowed by selecting ASG as the cladding material.

[0057] Through the mechanical optimization for the molten core method, high-quality glass-clad Si and Ge fibers were obtained. FIGS. 10A - 10X show the means of material characterizations indicate the polycrystalline nature of the fibers with limited oxygen content and free of cracks. Oxygen in the core may be resulted from thermally-activated dissolution and diffusion from the glass cladding. Extremely low oxygen semiconductor fibers can beachieved by introducing deoxidizer to the molten core method, and laser recrystallization can be applied when single crystal is desired.

[0058] FIG. 10A shows a schematic of a silicon (Si)Zsilica fiber under bending according to various embodiments. The Si / silica fiber shows flexibility. FIG. 10B shows a microscopy image of the fiber in FIG. 10A according to various embodiments. FIG. 10B illustrates that no crack / perturbation was observed in the Si core during bending. FIG. 10C shows a dark-field microscopy image of the polished transverse cross-section of the silicon (Si) / silica fiber according to various embodiments. FIG. 10D is a dark-field microscopy image of the polished lateral cross-section of the silicon (Si) / silica fiber according to various embodiments. FIGS. 10C - D demonstrate a dense core free of voids and cracks. FIG. 10E shows a scanning electron microscopy (SEM) image of a line scan that was applied to the transverse cross-section of the silicon (Si)Zsilica fiber according to various embodiments. FIG. 10F shows a plot of concentration (in atomic percent or at. %) as a function of relative distance (in micrometers) illustrating energy dispersive X-ray (EDX) results of a line scan that was applied to the transverse cross-section of the silicon (Si)Zsilica fiber according to various embodiments. FIG. 10G shows a plot of intensity (in arbitrary units or a.u.) as a function of angle 20 (in degrees) illustrating the X-ray diffraction patterns of the silicon (Si)Zsilica fiber according to various embodiments FIG. 10G may confirm the crystallinity of the resulting Si / silica fiber. FIG. 1 OH shows a high-resolution transmission electron microscopy (HRTEM) image of the silicon (Si)Zsilica fiber according to various embodiments, with the inset showing a selected area electron diffraction (SAED) image of the silicon (Si) / silica fiber according to various embodiments that confirms the [Fd3ml] space group. FIG. 101 shows a schematic of the germanium / aluminosilicate glass (Ge / ASG) fiber under bending according to various embodiments. FIG. 10 J shows a microscopy image of the fiber in FIG. 101 according to various embodiments FIG. 10J illustrates that no crack / perturbation was observed in the Ge core during bending. FIG. 10K shows a dark-field microscopy image of the polished transverse cross-section of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments. FIG. 10L shows a dark-field microscopy image of the polished lateral crosssection of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments FIGS. 10K - L demonstrate a high-quality core free of voids and cracks. FIG. 10M shows a scanning electron microscopy (SEM) image of a line scan that was applied to the transverse cross-section of a germanium / aluminosilicate glass (Ge / ASG) fiber according tovarious embodiments. FIG. ION shows a plot of concentration (in atomic percent or at. %) as a function of relative distance (in micrometers) illustrating energy dispersive X-ray (EDX) results of a line scan that was applied to the transverse cross-section of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments. FIG. 100 shows a plot of intensity (in arbitrary units or a.u.) as a function of angle 20 (in degrees) illustrating the X-ray diffraction patterns of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments. FIG. 100 may confirm the crystallinity of the resulting Ge / ASG fiber. FIG. 10P shows a high resolution transmission electron microscopy (HRTEM) image of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments, with the inset showing a selected area electron diffraction (SAED) image of the germanium / aluminosilicate glass (Ge / ASG) fiber according to various embodiments that confirms the [Fd3ml] space group. FIG. 10Q shows a microscopy image of a standalone silicon (Si) fiber of 55 pm diameter after the removal of glass cladding according to various embodiments. The standalone Si fiber remains after removal of the silica cladding of the Si / silica fiber. FIG. 1 OR is a schematic showing that the standalone silicon (Si) fiber according to various embodiments can be bent with a bending radius of 4 mm. FIG. IOS shows (left) a schematic illustrating that the standalone silicon (Si) fiber according to various embodiments can easily hold the weight of a 70 g computer mouse; and (right) a plot illustrating that the Si fiber according to various embodiments experiences a tensile strength of 338.39 ± 143.83 MPa when holding the computer mouse. FIG. 10T is a schematic showing that an 80 cm-long standalone silicon (Si) fiber according to various embodiments can be safely wrapped on a bobbin due to its mechanical strength. FIG. 10U shows a microscopy image of a standalone germanium (Ge) fiber of 55 pm diameter after the removal of glass cladding according to various embodiments. The standalone Ge fiber remains after removal of the ASG cladding of the Ge / ASG fiber FIG. 10V is a schematic showing that the standalone germanium (Ge) fiber according to various embodiments can be bent with a bending radius of 2.5 mm. FIG. 10W shows (left) a schematic illustrating that the standalone germanium (Ge) fiber according to various embodiments can easily hold the weight of a 86 g mouse; and (right) a plot illustrating that the Ge fiber according to various embodiments experiences a tensile strength of 505.85 ± 105.41 MPa when holding the computer mouse. FIG. 1 OX is a schematic showing that an 80 cm-long standalone germanium (Ge) fiber according to various embodiments can be safely wrapped on a bobbin due to its mechanical strength.

[0059] While the glass-clad semiconductor fibers are favored for optical use, it may be challenging to form well-defined interfaces between insulator, semiconductor, and conductor for in-fiber optoelectronic devices, mainly due to the limited control on the viscosities of these materials at high processing temperatures. Moreover, the mechanical properties of glass-clad fibers may be compromised due to the use of glasses. To address these difficulties of achieving flexible optoelectronic fibers, the standalone Si and Ge fibers shown in FIGS. 10Q - 10X may be obtained by removing the corresponding glass cladding using chemical etching. Next, these standalone semiconductor fibers may be combined with metal wires, conducting polymers, and insulating polymers to fabricate flexible optoelectronic fibers via convergence fiber drawing, by taking advantage of both high-quality semiconductors and high flexibility polymers. Intimate interfaces between insulators, semiconductors, and conductors may be formed at the necking region by the convergence fiber drawing, as shown in FIG. 3E.

[0060] In the design of an optoelectronic fiber device, a standalone Si or Ge fiber was placed in the center of a transparent polycarbonate (PC) cladding and sandwiched by two copper or tungsten wires. FIGS. 11A - 11C show how conducting carbon-loaded polycarbonate (CPC) was used to close the gap between semiconductors and metals.

[0061] FIG. HA is a microscopy image of the cross-sectional side view of a single-core optoelectronic fiber according to various embodiments, where the core semiconductor (silicon (Si) or germanium (Ge) is connected to each copper electrode through a layer of carbon-loaded polycarbonate (CPC) according to various embodiments. FIG. 1 IB is a microscopy image of the transverse cross-section of the single-core optoelectronic fiber in FIG. HA according to various embodiments. FIG. 11 C shows a schematic illustrating that approximately 50 meters of optoelectronic fiber according to various embodiments may be collected in a single draw. Back-to-back Schottky contacts were constructed at the transverse plane between CPC and semiconductors, and further connected with two metal buses to enable decent conductivity both across the microscale transverse plane and along the meter-scale fiber axis as shown in FIG. HD. FIG. 11D shows stimulated electric field distribution along the length of the optoelectronic fiber according to various embodiments, illustrating that the electric field along the fiber length is uniform. Although the Schottky contacts were established between CPC and semiconductors (back-to-back Schottky contacts), the structure may still be referred herein as metal-semiconductor-metal (MSM) for simplicity. The resulting optoelectronic fiber has arectangular cross-section of 300 gm in width and 200 gm in height with the MSM structure in the center, as shown in FIGS. 11 A and 1 IB.

[0062] FIG. 1 IE shows a plot of current (in micro-Amperes or gA) as a function of time (in seconds or s) illustrating that the resulting optoelectronic fiber according to various embodiments shows a pseudo-omnidirectional response that maintains sensitivity in different directions. FIG. HE shows the photo response from the Si optoelectronic fiber under front incident (perpendicular to the MSM structure) and the side incident (parallel to the MSM structure) 532 nm illumination, with a bias voltage of 2 V. The Ge optoelectronic fiber with a similar structure has a response that is similar as the Si optoelectronic fiber, but with an extended spectral response due to its smaller bandgap. The I-V characteristics of Si and Ge optoelectronic fibers are shown in FIGS 1 1F and 1 1 G. FIG. 1 IF shows a plot of current (in micro-Amperes or gA) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a silicon (Si) optoelectronic fiber according to various embodiments under different illumination conditions. FIG. 11G shows a plot of current (in micro-Amperes or gA) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristics of a germanium (Ge) optoelectronic fiber according to various embodiments under different illumination conditions.

[0063] A variety of other in-fiber structures may be studied. FIGS. 12A and 12B show a self-assembled p-n junction fiber. Self-alignment of the two semiconductor fibers (i.e. two cores) may be achieved via the size reduction of the polycarbonate cladding at the necking region during the convergence fiber drawing. FIG. 12A is a microscopy image of the cross- sectional side view of a dual-core p-n junction fiber according to various embodiments. FIG. 12B is a microscopy image of the transverse cross-section of the dual-core p-n junction fiber in FIG. 12A according to various embodiments. The I-V characteristic curve of the p-n junction fiber is shown in FIG 12C. This in-fiber self-assembly technique extends the accessible structures and increases the utility of fiber electronics. FIG. 12C shows a plot of current (in amperes or A) as a function of voltage (in volts or V) illustrating the current-voltage (I-V) characteristic of the p-n junction fiber according to various embodiments.

[0064] In terms of photo detecting performance, the Si optoelectronic fiber shows a responsivity of 0.16 ± 0.04 A W’1at 532 nm, while the Ge optoelectronic fiber shows a responsivity of 0.35 ± 0.07 A W’1at 1550 nm. The level of noise equivalent power (NEP) is 3.12 ± O.51xlO'10and 5.35 ± 1.97xl0'9W Hz1 / 2for the Si and Ge optoelectronic fibers,respectively. The two types of devices may respond in the order of microseconds (2.88 ± 0.72 and 0.99 ± 0.11 us for Si and Ge optoelectronic fibers respectively) and may have a 3dB bandwidth / , of 129.10 ± 34.89 kHz (Si) and 354.92 ± 34.67 kHz (Ge). FIG. 13A shows a schematic illustrating an overall performance evaluation of the resulting silicon (Si) and germanium (Ge) optoelectronic fibers according to various embodiments. The noise equivalent power (NEP) value of Si optoelectronic fibers is amplified by a factor of 10 for visualization. For measurements of responsivity and NEP, n = 9 for all cases. For other measurements, n = 6 for all cases. All data are presented as mean value ± standard deviation (SD). A comprehensive performance comparison of the optoelectronic fibers with previously published works and the commercial planar type photodetector is summarized in Supplementary Table 3 below. It is worth noting that both postprocessing techniques such as laser recrystallization and semiconductor manufacturing methods such as doping and lithography are applicable to the semiconductor fibers and may lead to enhanced performance.

[0065] The resulting optoelectronic fibers are mechanically robust. Stable photo responses were recorded with a bending radius as small as 5 mm and after repeated bending cycles (FIG. 13B). FIG 13B shows a plot of current (in arbitrary units or a u.) as a function of time (in seconds or s) illustrating the photo response of the optoelectronic fibers according to various embodiments before, during and after bending in cyclic testing, as well before and after wash testing. Further, the mechanical strength of the optoelectronic fibers was investigated. The tensile strength of the Si optoelectronic fiber (66.80 ± 10.74 MPa) and the Ge optoelectronic fiber (61.59 ± 6.38 MPa) may meet the requirement for manual and machine weaving. While tensile strength is the most common parameter of interest for fiber devices, the impact and torsional strengths may also be of concern, due to their relevance in wearable devices. The impact strengths of Si and Ge optoelectronic fibers using one of the most impact-resistant polymers (i.e., polycarbonate) as the cladding were measured to be 4.74 ± 1.86 and 4.93 ± 1.57MJ m'2respectively, indicating higher resistance to the breakage under sudden impact than some conventional fibers (see Supplementary Table 3 below). Moreover, the resulting optoelectronic fibers can withstand the torsional stress of 244.80 ± 70.94 and 272.99 ± 54.16 MPa before the break in twisting. FIG. 13C shows a microscopy image of twisted optoelectronic fibers according to various embodiments, which may still maintain functionality even with a turn density of three turns per mm. FIG. 13D shows recovered photoresponse of the fibers subjected to a compressive stress of 30 MPa after a night without any treatment,exhibiting the potential for underwater applications. FIG. 13D shows (above) a plot of pressure (in Mega-Pascals or MPa) as a function of time (in seconds or s) illustrating the pressure-time history of the fibers according to various embodiments during a compression test where a maximum compressive stress of 30 MPa was applied to the fibers according to various embodiments, and (below) a plot of current (in micro- Amperes or pA) as a function of time (in seconds or s) illustrating the photoresponse of the fibers according to various embodiments during the compression test. The maximum compressive stress is equal to an underwater pressure at 3000 meters. The dark current increased while the photoresponse decreased right after the compression. The performance recovered after a night without any additional treatment. FIG. 13E shows the washability of the optoelectronic fibers. FIG. 13E shows a schematic illustrating a functional fabric including fibers according to various embodiments being washed in a washing machine. FIG. 13F shows a thermal image indicating that the temperatures of the optoelectronic fibers according to various embodiments remained unchanged after five hours of continuous operation. Heat sinks may not be required even for hours long operation.

[0066] In addition to operating solely as single fiber devices, the optoelectronic fiber can be woven into large-scale fabrics, enabling a broad scope of applications. FIG. 14A shows a schematic illustrating the weaving process to form a functional fabric according to various embodiments. FIG. 14B shows a schematic illustrating a functional beanie including a functional fabric according to various embodiments. The functional fabric may allow the beanie to be used as an assistive apparel for outdoor use. The Ge optoelectronic fibers were interknitted in the beanie. The beanie may include a customized printed circuit board (PCB) of dimensions 48 mm by 32 mm, which may be used as the interface board that that acquires, processes, and transmits the signals from connected optoelectronic fibers to the smartphone. The PCB may be placed inside of the beanie tip Real-time monitoring of the photoresponse may be achieved. In the situation of the alerting sound interrupted by traffic noise or at crossings without the sound alerting module, the functional beanie may act as an assistive apparel for a visually impaired person. The signals from an infrared light source (e.g., traffic light) may be received by the beanie and transmitted to the smartphone. Then, the smartphone may inform the user whether it is the green light or red light through vibrations emitted by the smartphone.

[0067] FIG. 14C shows (botom left) a schematic illustrating a functional sweater including the functional fabric according to various embodiments which makes use of a light fidelity (Li- Fi) based indoor communication system to receive information of a particular building; (top left) a schematic of the functional sweater with the fiber grid according to various embodiments, and (right) a block diagram illustrating the receiving of data via the functional sweater according to various embodiments. FIG. 14C demonstrates that the integration of optoelectronic fibers on daily clothing can turn the passive apparel into a wearable receiver for light fidelity (Li-Fi) based indoor communication system. For instance, it has been demonstrated that a photo of a building (the Learning Hub at Nanyang Technological University) may be received via the functional sweater from a modulated light emitting diode (LED) light source, and a transfer speed of up to 40 KB / s may be achieved.

[0068] Furthermore, the optoelectronic fiber may enable wearable devices for healthcare. FIG. 14D shows (top left) a schematic illustrating a smartwatch with a watchband including the functional fabric for measuring heart pulses according to various embodiments; (botom left) a magnified schematic of the smartwatch according to various embodiments; and (right) plots of current (in arbitrary units or a.u.) against time (s) comparing results of a commercial sensor and a fiber according to various embodiments. A rigid planar-type photodetector (i.e., rigid sensor), nonconformal to the wrist, may be attached on the backside of commercial smartwatches or watchbands to monitor heart rate via photoplethysmography. The optoelectronics fibers may be woven into the watchband, turning the watchband into a flexible and conformal sensor. The optoelectronic fibers may achieve similar performance as commercial photodetectors, in addition to saving the tightly limited inner space of the watch.

[0069] The durability of the waterproof optoelectronic fibers under compression may allow the fibers to be suitable for underwater applications. FIG. 14E shows (left) a schematic of a fiber receiver array including the optoelectronic fibers as part of an underwater visible light communication system according to various embodiments; (bottom right) a side view schematic of a mini-submarine with optoelectronic fibers attached to form the array and coupled to a printed circuit board (PCB) according to various embodiments; and (top right) a top view schematic of the mini-submarine with optoelectronic fibers according to various embodiments. Eight fibers were conformally attached on the mini-submarine (every 45°, dividing the mini-submarine into 8 sections, each fiber representing a specific angle) to act as the receiver array (or grid) in an underwater visible light communication system. The printedcircuit board (PCB) was sealed in a plastic box. The PCB may allow for data visualization on a cell phone. The command line in the mobile application in the cell phone shows “Turn 135°” when the fiber at 135° receives the optical signal.

[0070] Various embodiments may relate to the mechanical design of the molten core method for preparing ultralong, fracture-free, and perturbation-free silicon and germanium fibers, which may lead to high-quality optoelectronic fibers that enable various applications. Various embodiments may relate to a broad scope of materials that may bring functional fibers one step further towards unprecedented sensing, actuating, energy converting, and computing capabilities, with applications in flexible and wearable devices to deliver innovations with economic and societal benefits.

[0071] Methods

[0072] Materials

[0073] Fused silica tubes (purity 99.99%) of 2.1 mm inner diameter (ID) and 12 mm outer diameter (OD) were purchased from Runtu Glass Products. Three sizes (2.1 mm ID and 3.6 mm OD, 9.1 mm ID and 11.3 mm OD, and 13.2 mm ID and 16 mm OD) of aluminosilicate glass tubes (SCHOTT 8253) and rods of 3 75 mm diameter were purchased from Schott AG. Undoped Si (purity > 99.999%, resistivity > 1.0 Q-cm) and Ge (purity > 99.999%) rods of 2 ± 0.127 mm diameter were purchased from Lattice Materials. P- and n-type Si rods (purity > 99.999%, resistivity < 0.02 Q-cm) of the same size were purchased from the same supplier. Carbon-filled polycarbonate (102-106 Q / sq) film (thickness 125 pm) was purchased from Boedeker Plastics. Copper (50 pm diameter) and tungsten (30 pm diameter) wires (purity 99.999%) were purchased from Xionglin Metals. Polycarbonate slabs (24x8x300 mm) were purchased from Tiannuo Polymers. Si and Ge rods were soaked in a 10% hydrofluoric acid solution to remove native oxide before use Pre-drying of polycarbonate and carbon-filled polycarbonate were done in a vacuum oven at 80 °C for 48 hours before being used in fiber drawing. Other materials were used without any further treatment.

[0074] Functional and characterization of semiconductor fibers

[0075] Si rod was inserted into a fused silica tube and sealed in vacuum (IxlO'2mbar) using an oxyhydrogen flame. Si / silica fibers were fabricated by drawing the preforms at 1950 °C via the molten core method, with the feed and draw rates set to be 0.002 and 3.2 cm / s, respectively. For Ge / ASG fibers, the fabrication started with preform assembling. The ASG tubes of 13.2 mm ID and 16 mm OD were drawn into thinner tubes with 8.9 mm ID and 11.1 mm OD at1190 °C. The ASG tubes of 9.1 mm ID and 11.3 mm OD were drawn into three sizes (6.7 mm ID and 8.8 mm OD, 4.9 mm ID and 6.6 mm OD, and 3.7 mm ID and 4 8 mm OD) through the same process. The 3.75 mm diameter ASG rod was drawn down to 2 mm diameter. The total five sizes of tubes were jacketed to form a preform of 2.1 mm inner diameter and 1 1.1 mm outer diameter, into which a Ge rod was inserted. ASG rods with 2 mm diameter were used in the vacuum (IxlO'2mbar) sealing of the assembly to finalize the preform. The preform was then drawn at 1150 °C to obtain Ge / ASG fibers with the feed and draw rate set to be 0.002 and 3.2 cm / s, respectively. No deoxidizer was used in the process. Standalone Si and Ge fibers were exposed by hydrofluoric acid etching. Before the etching process, glass-clad Si and Ge fibers were cut into segments with a length of 80 cm, limited by the dimension of the acid tank.

[0076] Raman spectra were collected using a Witec UHT S300 system (excitation wavelength 532 nm). Fiber lateral- and cross-sections were prepared with embedding samples in epoxy resin (EpoxiCure 2 and EpoThin 2) and then polished with 600, 1200, 2500, and 4000 grit silicon carbide grinding papers. Raman spectroscopy cannot precisely reveal the pristine stress value due to the necessary polish for sample preparation, but it can serve as a reference for qualitative analysis, as the precisely quantitative comparison between modelling (or simulation) and experimental results to determine the individual contribution of solidification expansion and thermal mismatch is currently challenging owing to the extreme working conditions. Through the mechanical optimization for the molten-core method, high-quality glass-clad Si and Ge fibers were obtained. Material characterizations indicate the polycrystalline nature of the fibers with limited oxygen content and free of cracks. Oxygen in the core results from thermally activated dissolution and diffusion from the glass cladding. Extremely low oxygen semiconductor fibers can be achieved by introducing deoxidizer, and laser recrystallization can be applied when the single crystal is desired. SEM and EDX measurements were conducted with the accelerating voltage of 20 kV and working distance of 10 mm using a JEOL JSM-7800F. X-ray diffraction data were extracted by the azimuthal integration of two-dimensional wide-angle X-ray scattering (WAXS) collected by a Xenocs Nanoinxider (sample-detector distance 79.84 mm, wavelength 1.54189 A, beam size 200 pm, exposure time 60 s, and Psi rotation at every 18°). The TEM lamellae were prepared by focused ion beam (FIB) milling. HRTEM and SAED images were collected with a double-tilt holder using a JEOL 21 OOF 200 kV field-emission transmission electron microscope.

[0077] Fabrication of optoelectronic fibers

[0078] Convergence fiber drawing is a modified thermal drawing technique that expands the material selections that are not limited by the process compatibility due to the drawing temperature. Using this method, the polymers transform into viscous flows and converge to the semiconductor fibers and metal wires that retain solidity when being drawn together down to the fiber dimension.

[0079] In the design of an optoelectronic fiber device, standalone Si or Ge fiber was placed in the center of a transparent polycarbonate cladding and sandwiched by two copper or tungsten wires, while conducting CPC was used to close the gap between semiconductors and metals. Back-to-back Schottky contacts were constructed at the transverse plane between CPC and semiconductors, and further connected with two metal buses to enable decent conductivity both across the microscale transverse plane and along the meter-scale fiber axis. The semiconductor core was slightly thicker than the opaque electrodes to enable pseudo-omnidirectional response to incident light with a beam size larger than the opaque electrodes. It is worth noting that both post-processing techniques, such as laser recrystallization, and semiconductor manufacturing methods, such as doping and lithography, are applicable to the semiconductor fibers and may lead to enhanced performance

[0080] Preparation of the preform used in convergence fiber drawing of optoelectronic fibers started with the milling of two PC slabs to create three hemispherical channels with a radius of 2 mm and 1 mm to each other in the center, along the preform length. Then, the PC in the 1 mm space between the three channels was machined off, and two 1 mm square CPC slabs were placed. The preform was then consolidated in a vacuum oven (3 hours at 170 °C). The preform was drawn at 300 °C, and the feed and draw rates were 0.002 cm / s and 3.2 cm / s, respectively. Copper wires (for single-core fiber) and tungsten wires (for dual-core fiber) were fed into the two side channels, while Si or Ge fiber (two fibers in the case of dual-core fiber) was fed into the center channel during the draw.

[0081] Simulations

[0082] The finite element analysis for the solidification and cooling stages was implemented with the software suite Abaqus / Standard. In all the simulations, axisymmetric structures were adopted to reduce computation cost. The parameters used in the simulations are listed in Supplementary Tables 1 and 4.

[0083] In the first stage, viscoelastic analysis was carried out for the stress distributions after solidification, and the glass claddings were considered as viscoelastic materials with theMaxwell model. The normalized shear relaxation modulus of the claddings was chosen as a value (0.99) close to one. The volume expansion of semiconductors at solidification was applied in the form of an isotropic eigenstrain induced by a virtual temperature change. As drawing a long fiber at constant velocity is a steady state process, the liquid-solid interface remained at the same position after At. Interfacial sliding was allowed over the portion of the solid core-cladding interface formed during At. The choice on the value of At does not affect stress evolution in the core if it is small compared with the stress relaxation time as shown in FIGS. 15A - 15F. The conclusions also hold for the other fibers with different ratios of core radius to fiber radius used in the experiment as shown in FIGS. 15A - 15F.

[0084] FIG. 15A shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor silicon (Si) core of a silicon (Si)Zsilica fiber for different fiber radii (rz) while keeping the same core radius (ri=50 pm) according to various embodiments. FIG 15B shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor silicon (Si) core of a silicon (Si)Zsilica fiber for different values of At according to various embodiments. FIG. 15C shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge)Zsilica fiber for different fiber radii (n) while keeping the same core radius (ri=50 pm) according to various embodiments. FIG. 15D shows a plot of stress (in giga- Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge)Zsilica fiber for different values of At according to various embodiments. FIG. 15E shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge)Zaluminosilicate glass (ASG) fiber for different fiber radii (rz) while keeping the same core radius (ri=50 pm) according to various embodiments. FIG. 15F shows a plot of stress (in giga-Pascals or GPa) as a function of time (in seconds or s) illustrating the evolution of maximum principal stress in the solidified semiconductor germanium (Ge) core of a germanium (Ge)Zaluminosilicate glass (ASG) fiber for different values of At according to various embodiments.

[0085] In the cooling stage, the viscous effect of the cladding was neglected, considering its much larger viscosities at low temperatures, and accordingly the cladding was modelled as a linear elastic material. Thermal residual stresses started to accumulate in the solidified core when the temperature reached the annealing point of the cladding (Tlad') above which the residual stresses would have been relaxed rapidly. Therefore, the thermal mismatch was calculated in the temperature range from T = min(T^>re, Talad)) down to the ambient temperature (26 °C). The total thermal strain in this temperature range was obtained by integrating the temperature-dependent data of the linear thermal expansion coefficient for Si, Ge, and SiCh. Both terminating cross-sections of the fiber were constrained such that the core and the cladding deformed synchronously in the axial direction. The effect of the drawing force is neglected, as it produces much smaller fiber tensions (10-20 MPa) than those induced by the solidification expansion and thermal mismatch.

[0086] Capillary instability calculation is described in Supplementary Note 2, and parameters used in the calculations are listed in Supplementary Table 5 Electric field distribution was calculated in the COMSOL electrostatics module with extremely fine mesh.

[0087] Performance characterization

[0088] A 532 nm laser diode (Thorlabs DJ-532) and a 1550 nm InGaAsP laser diode (NEC NX5504EK) were used as the illumination sources. Laser diode current controller (Thorlabs LDC205C), temperature controller (Thorlabs TED200C), and function generator (Agilent 33250A) were used to operate and modulate the illumination source. The laser beam was focused by a spherical lens and further shaped into an ellipse via a planoconcave lens to cover the whole aperture of optoelectronic fibers, which was placed in the beam center. Laser power was monitored by a power meter (Thorlabs PM100). In the measurement of responsivity and I-V curves, a source meter (Keithley 6517B) was used for power supply and current monitoring. Electrical connections to the fibers were established to the metal wire electrodes exposed by a fiber stripper. A transimpedance amplifier circuit (amplifier OP A380, load resistance 5k Ohm) was used for noise equivalent power and rise time measurements. Waveforms were collected through an oscilloscope (Tektronix DPO5104B), and MATLAB rise time and pwelch functions were employed. A bias voltage of 2V was applied to the fibers in all tests.

[0089] Linear translational stages (Thorlabs NRT150) were used in tensile and compression tests with a 20 N force gauge (Yisida DS2-20N) mounted. The impact strength was collectedfrom unnotched Charpy impact tests. Torsional strength testing was conducted on a customized testing machine. The cyclic bending test was performed on a PRTronic FT2000 flexible electronic tester. A commercial washing machine was used in the washability test. Following the TSO 6330 standard, 10 washing cycles were applied to the functional fabric.

[0090] Wireless optoelectronic fiber system

[0091] A 32 x 48 mm customized PCB was used as the interface board for data acquisition, processing, and wireless transmission modules. Two GS8554 (four channels) operational amplifiers were used in the transimpedance amplifier circuit, allowing the installation of eight optoelectronic fibers on the PCB. Analog-to-digital-converter (ADS7828) was used for onboard data processing, and the wireless transmission to cell phone by Bluetooth. A coin cell was used as a power supply A customized mobile application for visualization may be developed.

[0092] The functional beanie was achieved by interknitting with eight Ge optoelectronic fibers, which are connected to the PCB placed in the inner top space of the beanie. The demonstration of outdoor use was recorded at a pedestrian crossing at noon on a sunny day with the most intense sunlight of the day using a 30mW 1550 nm laser pointer as a signal source at the distance of 1.5 m to the beanie. Similarly, Si optoelectronic fibers were interknitted into a sweater and used to demonstrate a wearable receiver for an indoor Li-Fi communication system. A photo of the building (The Hive of Nanyang Technological University) was encoded by a customized algorithm into the light from an LED, which was received and converted into electrical signals by the sweater and decoded by a customized algorithm to restore the photo. Mini 532 nm LED strips and the Si optoelectronic fibers were integrated together into the watch band. A small portion of the green light related to the volume changes of blood vessels caused by heartbeats was scattered back from the wrist to the optoelectronic fiber and generated an electrical signal reflecting the heart rate. Conventional sensor BIOFY SFH 7070 was used in the same configuration for comparison. In the demonstration of underwater use, Si optoelectronic fibers were conformally (over right-angled steps) glued to the outer surface of the mini submarine. The PCB interface board was installed beneath the mini-submarine and protected by a water-proof plastic box.

[0093] Supplementary Information

[0094] Supplementary Table 1: Material properties of the aluminosilicate glass (from the technical manual provided by the supplier, the Schott company).

[0095] Supplementary Table 2: Stresses calculated from Raman spectra

[0096] Supplementary Table 3 : Comparison of performances of optoelectronic fibers in this work with previously reported optoelectronic fibers, commercial planar device, and conventional polymer and glass fiber material* Thorlabs FDS10X10

[0097] Supplementary Table 4: Parameters used in finite element simulations and the mechanical model for stress mismatch analysis

[0098] Supplementary Table 5: Parameters used in calculation of total growth factor

[0099] Supplementary Note 1 : Mechanical model for thermal mismatch in the fiber with a single core

[0100] An axisymmetric model was developed to consider the problem of thermal mismatch in a fiber with core and cladding structures. The core radius and the fiber radius (i.e., the outer radius of the cladding) are denoted as ri and ri, respectively. The core and cladding materials are linear elastic with Young’s moduli E:, Esand Poisson’s ratios vc, vswhere the superscript “c” represents the core and “s” the cladding.

[0101] When the core is in liquid phase or the temperature is above the annealing point of the cladding Talad, the stress induced by thermal mismatch can be rapidly relaxed and may not be accumulated in the subsequent drawing process. Therefore, the initial stress-free state is chosen as that at the temperature T = mm(T^re, T^lad) where T^oreis the melting point of the core. The total thermal strains EQ and Eg are evaluated from the initial temperature down to the ambient temperature. Tn the cooling stage, no relative displacement is allowed for the interfacebetween the core and cladding, and the cross-section of the fibre in the initial state is assumed to remain flat after thermal deformation. The core and cladding have a common strain EZin the axial direction, which, by neglecting the effect of drawing force, can be written as

[0102] where £edenotes the elastic strain in the axial direction. The net axial force on a fiber cross section should vanish, which yields (1.2)

[0103] where the cross-section areas of the core A =and the cladding As=- r^).From Eqs. (1.1) and (1.2), the axial strain of the fibre can be obtained as(1-3)

[0104] The normal strain components in the radial and tangential directions, denoted as Er and £e, respectively, can be divided into the thermal part and elastic part resembling Eq. (1.1). Considering the relation= £Q in this axisymmetric problem, the radial stress in the core can therefore be obtained from the elastic constitutive equations as(1-4)

[0105] Since stress and strain are uniformly distributed over the cross section of the core, the normal traction and radial displacement of the core at the core-cladding interface are

[0106] The cladding embedding the deformed core can be considered as an elastic tube under a uniform inner pressure pint whose value is to be determined By adopting the Airy stress function method, the stress fields for such problem can be obtained as

[0107] where A, B and C are constants. Note that the cladding has a total strain of Ez in the axial direction as the core does. Therefore, based on Eqs. (1.6), the radial and tangential strain components of the cladding can be calculated as(1-8)

[0109] where F is a constant. Since the tangential displacement u9s (r, 0) = 0,[001 10] which results in B = E = 0. Therefore, Eqs (1 .6) and (1 .8) become

[0111] On the core-cladding interface, the distributions of normal displacement and stress should be continuous, that is, u*(r = rj = uint, (1.11a)Or O = O) = Pint- (1.11b)

[0112] In addition, the outer surface of the cladding is stress-free in the radial direction: a^(r — r2) = 0. (1-12)

[0113] Eqs. (1.5), (1.6), (1.9) and (1.10), together with boundary conditions Eqs. (1.11) and (1.12), give closed-form solutions to the distributions of displacements and stresses of bothfiber components. From the constitutive equations, the axial stress components of the core and cladding can be obtained as

[0114] where “x” represents “c” or “s” .

[0115] Supplementary Note 2: Calculation of capillary instability

[0116] Drawing a fiber creates a necking-down region in the heating zone. Peaked at the necking-down region, a convex temperature profile is established along the drawing direction, which softens the amorphous cladding material and melts the core material at a certain length. However, current theories on capillary instability of cylindrical threads may not be applicable due to the dimension reduction in necking-down region. A neck profile may need to be considered in the modelling. The calculation of the capillary instability for the cylindrical case is first described, followed by how the neck profile is calculated and applied into the calculation of capillary instability in the necking-down scenario.

[0117] Consider an incompressible viscous fluid fiber with a viscosity of p and density of p for the case in which the motion is symmetrical about the fiber longitudinal axis z. A perturbation in radius is assumed to be proportional to eint, where t is the time The perturbation is described spatially along the z axis by eikzwith k as the wavevector. The equations of motion for velocity (u, v, w) in the cylindrical coordinate system are:

[0118] where p is the pressure, and the equation of continuity is:

[0119] The Stokes current function ip is used to describe the flow velocity in a three- dimensional incompressible flow with axisymmetry in fluid dynamics. It is introduced to satisfy the equation of continuity since the fluid is assumed to be incompressible,1 dip 1 dip u — - , w = - (2.3) r dz r dr

[0120] Eliminating p in the equations of motion and introducing the Stokes current function leads to:

[0121] where D denotes the differential operator:

[0122] The squares and products of velocity components may be neglected with an assumption of slow motions in Eq. (2.4), and it is reduced to a simpler form:

[0123] The function ip can be separated into two parts, ip\ and ip2, since the operators D -- T) 7 d7t- and D are commutative with each other:

[0124] With the assumption of the motions being proportional to eintand e‘kz:

[0125] Bothare functions of r only. By combining Eqs. (2.5), (2.7), and (2.8), the equations for <bi and <Pi become:

[0126] where

[0127] The general solution to Eq. (2.9) is: (fcr), (2.11a)(k1r), (2.11b)

[0128] where / n(x) and Kn(x) are the n-th order modified Bessel functions and An, Bnare constants The general solution to Eq. (2.6) becomes:= {[Ajrl^kr) + B1r / 1(fcr)]

[0129] The core fluid has a viscosity of ifcore, density of pCore, and radius of ri, while the infinitely thick cladding has a viscosity of r / d.-id and density of pciad. The wavelength 2 of varicosity (the growth of instability) is in the relationship with k by 2 = 2n'k For the core fluid, since K"i(0) —>■ co we have:

[0131] and for the cladding, since( ) — o we have:)

[0132] where ki is given by Eq. (2.10).

[0133] In Eqs. (2.13) and (2.15), the constants Ai, A2, B i and B2are determined by boundary conditions. Three boundary conditions at the interface of the two fluids are:

[0134] 1 . There is no slipping between the two fluids at the interface. At the interface (r=ri) the velocity components are continuous:(2.16)

[0135] 2. The shear stress is continuous at the interface (r=n):

[0136] 3. Interfacial surface tension contributes to the difference in the normal stress at the interface (r=n):

[0137] where y is the interfacial tension between the two liquids, and p = -p + 2p — is the normal stress (of the core / cladding) at the interface.

[0138] The boundary conditions together with Eqs. (2.13) and (2.15) result in a system of linear equations in terms of the constants Ai, Ai, Bi and B2. Nontrivial solutions require the determinant of coefficient matrix to be zero. By neglecting the inertia effect and considering the limit of p determinant becomes= 0 (2.19)

[0139] where

[0140] The value of n can be determined as a function of kri by this equation. Neglecting the effects of inertia, expanding the determinant in Eq. (2.19) with respect to the fourth row and solving for in,: (fcri). (2.21)

[0141] Here <p(kn) is treated as the growth factor of instability and given by:

[0142] where

[0143] In Eq. (2.23), An is function of kri in determinantal forms as:

[0144] The instability growth factor <p(kn) can be determined using Eq. (2.22).

[0145] The wavelength of corresponding instability growth is A = 2n / k. The instability is suppressed when A is less than 2nri (kri larger than 1). Thus, kri in the range of 0 to 1 is studied. The actual breakups are determined by the instability with maximum growth factor, and the final spacing of the resulted spheres is the wavelength corresponding to the maximum instability. Therefore, the breakups occur with the wavelength depending on the viscosity ratio and the core diameter.

[0146] The calculation of instability growth factor for a core-clad fiber system is described above. However, the fiber dimension, viscosity, velocity, and temperature vary around the necking region during the thermal drawing process. A dynamic model needs to be considered to raise a criterion for the capillary breakup instability in the thermal drawing process. Due to cross section change induced by necking down, the maximum instability at each z position is different, and the cumulation of instability contributions over the necking region should be considered. As the fiber stays in a viscous state when dwelling time in the furnace, r = 1 / in is defined as a characteristic time for instability to grow. For exponential instability growth, the perturbation amplitude £ evolves following= E / T. Integrating the equation with respect to <- dt time t, the total growth is £ oc eTltl. The velocity w is position-dependent in thermal drawing and there is Az = iv('z)dt The total instability growth factor % can be obtained by:

[0147] where z = 0 is the upper start of the necking region and z = L is the end of the necking region where the final fiber size is reached. The T(Z) is the shortest growth time at each axialposition of the necking region and is calculated by the inverse of Eq. (2.21). The case of a total instability growth factor much smaller than 1 is considered that no capillary instability develops in the draw.

[0148] To calculate the total instability growth factor / , the neck profile R(z) and the distributions of temperature, viscosity and velocity along the axial position are required. A modified physical model may be considered where the quantities of interest can be obtained by iterative calculation from the coupled momentum and energy equations for a given set of drawing parameters. The influence of core material on the neck profile is neglected due to the small volume ratio of core material, and it may be assumed that the radial component of velocity is negligible in comparison with the axial velocity.

[0149] Take the starting and finishing point of neck down region as z = 0 and z = L, the force balance equation can be expressed when the force from surface tension is balanced with the force normal to the surface:(2.26)

[0150] where n is the unit vector, S the stress tensor, yo the surface tension and H the mean curvature of the surface at r = / ?(z), defined by: (2.27)

[0151] where Eq. (2.26) can be written as:(2.28)

[0152] where nrand nzare the radial and axial components of the unit vector n, expressed as:

[0153] As the Reynolds number is small, the force balance equation can be written by neglecting the inertial terms as:(2.30)

[0154] where g is the gravitational constant. At the starting and finishing points of neck down region, the boundary conditions may be:

[0155] Multiplying Eq. (2.30) by 2 m' dr, integrating from 0 to R(z), and assuming the elongational and Newtonian flow model with Szz—

[0156] Applying the boundary condition in Eq. (2.31) to the double integration of Eq.

[0157] The heat flux leaving the furnace may be partially absorbed by the preform in the neck down region. While some of the absorbed energy dissipates to the surroundings, part of it may be conducted through the neck down region. Thus, the nonradiative thermal conductivity Kc and radiative thermal conductivity Krmay contribute together to the apparent thermal conductivity K. The radiative thermal conductivity Kr is expressed by:

[0158] where <r is the Stefan-Boltzmann constant, no the refractive index, and a the absorption coefficient. We neglect the transverse temperature gradient as the diameter of preform is less than 15 mm. The heat fluxand heat conduction energy balance in the z direction can be expressed by:dT

[0159] where A(z) = nR (z) is the area of cross-section, T' = —the temperature gradient, CPthe specific heat capacity, E the emissivity, To taken to be half of the highest furnace temperature, and qr-nthe heat flux from furnace surface to preform surface, qr-n is calculated bythe integration of the product of emissivity, radiative flux J and shape factor, over the furnace surface:

[0160] where rfnis the radius of furnace chamber, Lfnthe furnace length, and I the length of the section of furnace above the necking region Radiative flux J can be calculated by:

[0161] where the axis j for furnace is parallel to z and originates at the beginning point of the necking region. The constant C is obtained from the temperature profde of the furnace, and Jo can be calculated by:

[0162] where £& is the emissivity of the furnace and Tmax is the highest furnace temperature. [00163

[0164] where Ro denotes the radius of the preform. From Eq. (2.40), it can be deduced that q' is:

[0165] Then Eq. (2.41) becomes:

[0166] Substituting T = Ti and= T2into Eq. (2.43), a system of first-order equations for numerical implementation may be obtained:(2.44a)(2.44b)

[0167] The boundary conditions are:

[0168] To calculate the neck profile, a hyperbolic tangent function may be first used as the input:, r ) =2RR(zo+ tanh(4C) tanh[4(z® — C)]0tanh[4(LB— C)]+ tanh

[0169] where A, B and C are constants. The temperature profile T(z) can be calculated fromEq. (2.44). From T(z), the viscosity profile t](z) may be obtained using the temperatureviscosity relationship of the cladding material. Eq. (2.33) provides a velocity profile w(z), from which a new neck profile may be obtained through the law of conservation of mass. The new neck profile is again used as the input for next calculation, until convergence is reached. The final neck profile is then used in the calculation of total growth factor in Eq. (2.25). MATLAB bvp5c is used to solve the boundary value problem. Less than ten iterations are usually sufficient, depending on the initial input. To verify the model, the neck profile of a silica preform is calculated, with preform feed and fiber draw speed set as 0.001 and 14.4 cm / s, draw temperature set as 1950 °C as shown in FIG. 9B. Other parameters used in the calculation are listed in Supplementary Table 5. For E and h, two values are used for the preform and fiber region, respectively. The first value is used for the section that has a diameter larger than 0.4 cm, and the second value is used for the section with fiber diameter. In the intermediate section, a polynomial is used to smoothly connect the two values. The comparison of the calculated neck profile and experimental result is shown in FIG 9C. The preform was drawn with the same set of draw parameters and then quenched immediately after lifted out from the furnace to maintain the neck profile. Some cracks formed at quenching, which did not affect the result.

Claims

Claims1. A method of forming a semiconductor fiber, the method comprising: determining, via a computer, a suitable combination of a fiber core material and a cladding material; providing the fiber core material into an inner cavity of a tube, the tube comprising the cladding material, to form a composite structure; and subjecting the composite structure to a thermal drawing process at a draw temperature higher than a melting point of the fiber core material to form the semiconductor fiber comprising a core comprising the fiber core material, and a cladding around the core, the cladding comprising the cladding material; wherein the suitable combination of the fiber core material and the cladding material is determined based on a closeness of a softening point of the cladding material and the melting point of the fiber core material, a closeness of a coefficient of thermal expansion (CTE) of the cladding material and a coefficient of thermal expansion (CTE) of the fiber core material, and a total growth factor of the composite structure at the draw temperature and based on provided parameters of the core of the semiconductor fiber.

2. The method according to claim 1, wherein the provided parameters of the core of the semiconductor fiber comprise a diameter of the core and a geometry of the core.

3. The method according to claim 1 or claim 2, wherein the fiber core material undergoes a melting process, followed by a solidification stage and a cooling stage during the thermal drawing process.

4. The method according to any one of claims 1 to 3, wherein the suitable combination of the fiber core material and the cladding material is determined by requiring the total growth factor to be below 1.

5. The method according to any one of claims 1 to 4, wherein the suitable combination of the fiber core material and the cladding material is determined by requiring the softening point of the cladding material to be 50 °C to 200 °C higher than the melting point of the fiber core material.

6. The method according to any one of claims 1 to 5, wherein the suitable combination of the fiber core material and the cladding material is determined by requiring that a difference between the coefficient of thermal expansion (CTE) of the cladding material and the coefficient of thermal expansion (CTE) of the core material to be below 2.8 - 1 O'6K4.

7. The method according to any one of claims 1 to 6, wherein the total growth factor is determined based on a temperature distribution, a viscosity distribution and a velocity distribution along an axial position of a necking region of the composite structure at the draw temperature and provided parameters of the core of the semiconductor fiber.

8. The method according to any one of claims 1 to 7, wherein the total growth factor is determined via iterative calculations.

9. The method according to any one of claims 1 to 8, wherein the fiber core material is any one selected from a group consisting of silicon (Si), germanium (Ge), silicon-germanium (SiGe), gallium arsenide (GaAs), gallium antimonide (GaSb), gallium nitride (GaN), and silver sulfide (Ag2S).

10. The method according to any one of claims 1 to 9, wherein the fiber core material is germanium and the cladding material is aluminosilicate glass (ASG)11. The method according to any one of claims 1 to 9,where the fiber core material is silicon and the cladding material is silica (SiO2).

12. The method according to any one of claims 1 to 1 1 , wherein an outer diameter of the tube is greater than an outer diameter of the cladding.

13. The method according to any one of claims 1 to 12, wherein the core of the semiconductor fiber formed is devoid of cracks.

14. A method of forming an optoelectronic fiber, the method comprising: forming one or more semiconductor fibers according to any one of claims 1 to 13; removing the cladding of each of the one or more semiconductor fibers to expose the one or more cores; and subjecting the one or more cores with one or more metal wires, a first polymeric material and a second polymeric material to a convergence fiber drawing process to form the optoelectronic fiber.

15. The method according to claim 14, wherein the cladding of each of the one or more semiconductor fibers is removed by chemical etching.

16. The method according to claim 14 or claim 15, wherein the optoelectronic fiber comprises: a polymeric intermediate layer comprising the first polymeric material surrounding the one or more cores; the one or more metal wires substantially parallel to the one or more cores such that the polymeric intermediate layer is between the one or more cores and the one or more metal wires, anda polymeric outer layer comprising the second polymeric material around the one or more metal wires.

17. The method according to any one of claims 14 to 16, wherein the first polymeric material is any suitable conductive thermoplastic material.

18. The method according to claim 17, wherein the suitable conductive thermoplastic material comprises carbon- loaded thermoplastic material, liquid metal-loaded thermoplastic material or silver parti cl es-loaded thermoplastic material19. The method according to claim 18, wherein the suitable conductive thermoplastic material comprises the carbon- loaded thermoplastic material; and wherein the carbon-loaded thermoplastic material comprises carbon-loaded polycarbonate (CPC) or carbon-loaded polyethylene.

20. The method according to any one of claims 14 to 19, wherein the second polymeric material is any suitable thermoplastic material.

21. The method according to claim 20, wherein the suitable thermoplastic material is polycarbonate (PC), polyethylene, polystyrene, cyclic olefin copolymer, or polyetherimide.

22. The method according to any one of claims 14 to 21, wherein the one or more metal wires comprise copper, tungsten, titanium, nickel, aluminum, steel, zinc, brass, silver, or gold.

23. The method according to any one of claims 14 to 22, wherein the one or more cores comprise a first p-doped core and a second n- doped core.

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