Method for measuring electronic spin lifetime of semiconductor colloidal nanomaterials based on doping method

Abstract

The application relates to a method for measuring the electron spin lifetime of semiconductor colloidal nanomaterials based on a photochemical charge doping method. As a light absorber, the semiconductor colloidal nanomaterials are excited under ultraviolet light to generate excited state electrons and holes on a conduction band and a valence band respectively. A strong reducing agent lithium triethylborohydride (LiEt3BH) is used as an electron dopant to capture the photo-generated holes on the valence band of the semiconductor colloidal nanomaterials, and the electrons on the semiconductor conduction band cannot return to the ground state, thereby loading negative charges, that is, the photochemical doping of the electrons is completed. On this basis, the electron spin relaxation lifetime is measured based on the semiconductor carrier optical transition selection rule and the circular polarization transient absorption spectrum (TA) technology. The method for measuring the electron and hole spin lifetimes in the semiconductor based on the charge doping method is a direct way to accurately obtain the carrier spin lifetime, and lays an important foundation for developing and utilizing the charge spin properties of semiconductor materials.

Description

TECHNICAL FIELD

[0001] The present application relates to a method for accurately measuring charge spin lifetime by combining optical means after semiconductor colloidal nanomaterial is doped with charge. BACKGROUND

[0002] In recent years, quantum technology has developed rapidly, and accelerating the strategic layout of China's high-end technology to occupy the international competitive commanding point of quantum technology has become a major development goal of China. Semiconductor spintronics technology is the basis for the development of quantum technology dominated by quantum communication and quantum computing. The key to the research and development of semiconductor spintronics technology lies in carrier spin injection and spin relaxation dynamics detection. Semiconductor materials have a specific optical band gap, and the absorption of photons is selective. For example, light smaller than the band gap is "transparent"; in addition, the interband transition of carriers follows the optical transition selection rule, so that circularly polarized light can be used for electron spin injection and spin relaxation testing and research. Therefore, it is of great scientific significance and broad application prospects to develop semiconductor materials with excellent optical properties, efficiently inject spin-polarized current, accurately measure carrier spin relaxation lifetime, and deeply understand the spin relaxation micro mechanism to realize the regulation of spin relaxation.

[0003] As an important product of semiconductor band engineering, semiconductor colloidal nanomaterials have a volume between bulk materials and molecules, but their physical properties are neither simply volume reduction of bulk materials nor simple combination of molecular systems. Due to its unique quantum confinement effect, it exhibits size-dependent physical properties and photoelectric properties. Its optical, electrical, magnetic and other properties have attracted much attention and have important applications in photovoltaics, lasers, photodetectors, biomedicine and many other fields.

[0004] In recent years, interesting optical phenomena such as optical Stark effect and optical circular dichroism effect closely related to the electronic spin motion properties of semiconductor colloidal nanomaterials have predicted a broad application prospect of nanometer semiconductor materials in the field of spintronics. Based on the spin regulation of semiconductor colloidal nanomaterials, spin valve, circularly polarized light source and other devices have also been proposed. However, the current actual performance is poor and the research and development progress is slow, which is mainly due to the relatively lagging behind of the basic research on the carrier spin properties of semiconductor colloidal nanomaterials. At present, the basic research on the carrier spin properties of semiconductor nanosystems reported in the literature is still in its infancy, mainly focusing on the measurement of spin lifetime. Benefiting from the excellent optical properties of semiconductor colloidal nanomaterials, the most commonly used measurement method is to perform optical detection of spin relaxation on the basis of optical injection of spin. Common optical detection techniques include time-resolved transient absorption (TA) spectroscopy, time-resolved Kerr rotation (TRKR) and time-resolved Faraday rotation (TRFR) techniques. Based on these testing methods, the spin relaxation of semiconductor colloidal nanosheets, nanocrystals, quantum dots and other samples has been studied, but the reported spin lifetimes span the time scale from sub-picoseconds to nanoseconds, and there is a lot of controversy about the relaxation rates of electrons and holes. This is partly due to the uneven quality of the sample morphology, and another important reason is that the exchange interaction between electrons and holes in the quantum confined system is enhanced and the strong spin-orbit coupling (SOC) makes it very difficult to distinguish the signals of electrons and holes, which directly affects the accuracy of the measurement of electron and hole spin lifetimes.

[0005] We designed and synthesized stable monodisperse CdSe / ZnS core-shell colloidal nanosheet samples (CdSe / ZnS NPLs), and completed the electron doping of CdSe / ZnS NPLs under ultraviolet light irradiation by using strong reducing agent LiEt3BH as a hole trapping agent. As shown in Figure 1 As shown in the charge doping schematic diagram, we selectively doped the nanosystem with negative charges and then realized the separate measurement of electron spin lifetime based on the optical transition selection rule as shown in Figure 3 (c). As can be seen from Figure 2 , this testing method not only eliminates the influence of holes on the excited state electrons but also does not need to deeply separate and extract the signals of electrons and holes, thereby ensuring the accuracy of the measurement results. This invention provides an important basis for future research on the carrier spin properties of semiconductor colloidal nanomaterials and lays a prerequisite for the commercialization of semiconductor spintronic devices. SUMMARY

[0006] The purpose of the present application is to provide a method for measuring the electron spin lifetime of semiconductor colloidal nanomaterials based on charge doping, so as to solve the technical problem of inaccurate measurement of electron spin lifetime in semiconductor colloidal nanomaterials.

[0007] The doping method is based on photochemical charge transfer between a strong reducing agent and the semiconductor colloidal nanomaterial.

[0008] The semiconductor colloidal nanomaterial can be composed of a compound of zinc, cadmium or mercury in the second sub-group of the periodic table and sulfur, selenium or tellurium in the fifth main group, indium phosphide, lead selenide, lead telluride and two-dimensional nanosheet of trihalogen perovskite (with ABX3 structure, wherein A=C, CH3NH3 or CH(NH2)2; B=Pb or Sn; X=one or more than two of Cl, Br and I), three-dimensional spatially confined quantum dots and one-dimensional nanorods, etc.

[0009] The semiconductor colloidal nanomaterial can be a homogeneous nanomaterial or a core / shell heterostructure colloidal nanomaterial.

[0010] The electron dopant is a strong reducing agent LiEt3BH.

[0011] The doping condition is continuous irradiation of light smaller than the optical band gap of the semiconductor material.

[0012] The electron spin lifetime test technology is a circular polarization transient absorption spectrum technology.

[0013] The semiconductor colloidal nanomaterial is preferably a CdSe / ZnS core / shell nanosheet with stable chemical properties; the core / shell heterojunction colloidal nanosheet is simple to prepare, has high stability and high charge doping efficiency, and is expected to be used for preparing a spin electronic device with excellent performance in the future.

[0014] In order to verify whether the charge doping measurement of the electron spin lifetime can realize accurate measurement of the electron spin lifetime, the verification technical scheme adopted by the present application is that:

[0015] The basic light absorption, emission characteristics and fluorescence quantum efficiency of the CdSe / ZnS core / shell colloidal nanosheet are determined by using steady-state absorption and fluorescence spectrum.

[0016] Based on the steady-state absorption spectrum data before and after the charge doping, the electron doping amount is calculated, and the circular polarization transient absorption spectrum is tested, and the electron spin lifetime is fitted by the difference of the band edge dynamics in the excited-probe different direction circular polarization combination state.

[0017] Compared with the prior art, the present application has the beneficial effects that:

[0018] 1. The method for measuring electron spin lifetime of the photochemically charge-doped semiconductor material according to the present application makes nanosheets selectively carry negative charges, only the signal of electron spin relaxation in spin relaxation dynamics is measured, the interference of hole spin relaxation on the signal of electron spin relaxation can be effectively avoided, and the electron spin relaxation lifetime can be accurately measured.

[0019] 2. The present application provides a direct means for accurately measuring the carrier spin lifetime of semiconductor colloidal nanomaterials in the future, and lays an important foundation for the development and application of semiconductor spin properties. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 . Schematic diagram of photochemical charge doping principle of semiconductor colloidal nanomaterials;

[0021] Figure 2 . Transient dynamics process of charge-doped semiconductor colloidal nanomaterials excited by circularly polarized light;

[0022] Figure 3 (a) Steady-state UV-Vis absorption and fluorescence spectra of CdSe / ZnS core / shell colloidal nanosheet toluene dispersion; (b) UV-Vis absorption spectra of charge-doped and de-doped CdSe / ZnS core / shell colloidal nanosheet toluene dispersion; (c) Band edge exciton fine structure and optical transition selection rule schematic diagram of CdSe / ZnS core / shell colloidal nanosheet;

[0023] Figure 4 (a) Transient absorption spectra of CdSe / ZnS core / shell colloidal nanosheet measured by dropping 20 μL electron dopant under the same circular polarization direction of pump-probe laser in transient absorption spectrum test after 15 minutes of UV light reaction; (b) Transient absorption spectra of CdSe / ZnS core / shell colloidal nanosheet measured by dropping 20 μL electron dopant under the opposite circular polarization direction of pump-probe laser after 15 minutes of UV light reaction;

[0024] Figure 5 (a) Transient absorption spectra dynamics of CdSe / ZnS core / shell colloidal nanosheet after 15 minutes of UV light reaction by dropping 20 μL electron dopant; (b) Electron spin relaxation dynamics of charge-doped CdSe / ZnS core / shell colloidal nanosheet. DETAILED DESCRIPTION

[0025] The present application is further illustrated by examples and drawings.

[0026] EXAMPLE

[0027] The method for measuring electron spin lifetime of the photochemically charge-doped semiconductor colloidal nanomaterials according to the present application, the charge doping method comprises the following steps:

[0028] A 0.1 mL, 1 mol / L LiEt3BH tetrahydrofuran solution was diluted to 0.01 mol / L with 9.9 mL of toluene solution in a nitrogen box. A toluene solution (1 mL) of CdSe / ZnS core-shell nanosheets with a first exciton absorption intensity of 0.4 OD (wherein the CdSe core is a cuboid nanosheet with a length of 15 nm, a width of 10 nm, and a thickness of 1.4 nm, and after being coated with a ZnS shell layer, it is a cuboid nanosheet with a length of 18 nm, a width of 13 nm, and a thickness of 2.0 nm) was placed in a nitrogen box and stirred for 20 minutes, and after the oxygen in the solution was completely removed, 20 μL of the diluted LiEt3BH tetrahydrofuran-toluene solution was added dropwise into 0.5 mL of the toluene solution of the CdSe / ZnS core-shell nanosheets, and after the dropwise addition was completed, the solution was continuously irradiated with a 365 nm ultraviolet lamp for 15 minutes.

[0029] In order to verify that the reducing agent LiEt3BH successfully doped electrons into the CdSe / ZnS core-shell nanosheets, steady-state and transient spectrometers were used for verification, specifically in the following two aspects:

[0030] (1) Steady-state absorption and fluorescence spectra of CdSe / ZnS colloidal nanosheets

[0031] Using a steady-state absorption spectrum detection method, the steady-state absorption and fluorescence characteristics of the undoped CdSe / ZnS colloidal nanosheets were first tested, wherein the ultraviolet-visible steady-state absorption spectrum was obtained using an Agilent Carry 5000 instrument, and the fluorescence spectrum was obtained using an Agilent Cary Eclipse fluorescence spectrophotometer with an excitation wavelength of 350 nm, as shown in Figure 3 (a), which is the steady-state absorption and fluorescence spectrum of the sample used in the present application, the steady-state absorption peak is located at 576 nm, and the fluorescence peak is located at 586 nm.

[0032] (2) Steady-state absorption spectrum of charge-doped and de-doped CdSe / ZnS colloidal nanosheets

[0033] After the toluene solution of the charge-doped CdSe / ZnS colloidal nanosheets sealed in a 1 mm closed cuvette was taken out of the nitrogen box, the steady-state absorption spectrum was tested using an Agilent carry 5000 instrument. As shown in Figure 3 (b), the dashed line is the steady-state absorption spectrum of the sample after charge doping. The cuvette cap was unscrewed and placed in the air and the cuvette was gently shaken to allow the charge-doped sample to fully contact with oxygen and be de-doped. After 15 minutes of contact with air, the steady-state absorption spectrum was tested again, as shown in Figure 3(b) shows, the steady-state absorption spectrum intensity of the de-doped sample is obviously enhanced. The absorption intensity of the sample after charge doping is obviously weaker than that of the de-doped sample, which is the result of the weakening of the inter-band transition intensity of the electrons due to the successful doping of the sample with charges, thus the sample is successfully doped with charges.

[0034] (3) Measurement of the electron spin lifetime of the charge-doped CdSe / ZnS colloidal nanosheet

[0035] The toluene solution of the successfully charge-doped CdSe / ZnS colloidal nanosheet was sealed in a 1mm closed cuvette and placed on the sample holder of the transient absorption spectrometer, and the transient absorption spectrum was tested with circularly polarized excitation light and circularly polarized probe light, and the energy of the circularly polarized excitation light was equal to that of the band edge exciton, and here the circularly polarized excitation light of 575nm was selected. During the test, the circular polarization direction of the probe light was fixed, and the circular polarization direction of the excitation light was changed, as shown in Figure 4 (a) and (b) show the test results when the circular polarization directions of the excitation light and the probe light are the same and opposite, respectively. It can be seen from the figure that when the circular polarization combination of the excitation light and the probe light is the same (co-polarization) and opposite (counter polarization) during the test, the spectral shapes are different, which is determined by the selection rule of the optical transition of the electrons. The former obtained during the test is the spectral change process caused by the spin-up electron inversion; the latter is the spectral change characteristics of the spin-down state electron generation process. The spin relaxation dynamics extracted from the transient absorption spectrum test results is shown in Figure 5 (a), when the circular polarization combination of the excitation light and the probe light is the same during the test, the dynamics is fast decay, and when the circular polarization combination of the excitation light and the probe light is opposite during the test, the dynamics is slow generation. The result obtained by subtracting the dynamics of the excitation light and the probe light in the same direction from that in the opposite direction is the electron spin relaxation dynamics, as shown in Figure 5 (b), we obtained the electron spin relaxation lifetime of the doped sample as 143.7ps.

[0036] The application is a method for measuring electron spin lifetime in semiconductor colloidal nanomaterials based on charge doping. The semiconductor colloidal nanomaterials are excited by light to generate electron-hole pairs filled in the conduction band and the valence band, respectively. The holes in the valence band of the semiconductor are captured by using a strong reducing agent molecule as an electron donor to achieve charge doping to make the semiconductor material carry negative charge. Then, based on the optical transition selection rule, the electron spin relaxation spectrum and dynamics are tested by using a circular polarization transient absorption spectrometer, and the electron spin relaxation lifetime in the semiconductor colloidal nanomaterials is obtained. The method selectively performs charge doping to make the semiconductor material carry a single charge, and the overlapping of the electron and hole spin relaxation signals caused by the strong interaction between the electrons and holes is excluded, so that the carrier spin signal is difficult to distinguish, thereby ensuring the accuracy of the electron spin lifetime measurement. The experimental scheme is simple to operate. The preferred scheme is that CdSe / ZnS core / shell nanosheets are used as semiconductor materials, a reducing agent LiEt3BH tetrahydrofuran-toluene diluent is used as an electron dopant, and the electron-doped nanosheets are obtained under 365 nm ultraviolet light irradiation; and the electron spin lifetime is accurately obtained in combination with the transient absorption spectroscopy technology.

Claims

1. A method for determining the electron spin lifetime of semiconductor colloidal nanomaterials based on doping, specifically a method for determining the electron spin lifetime of semiconductor colloidal nanomaterials based on photochemical charge doping, characterized by: The aforementioned photochemical charge doping method is charge doping completed under illumination based on charge transfer during a photochemical reaction between semiconductor colloidal nanomaterials and electronic dopants; the electron spin lifetime of the charge-doped semiconductor colloidal nanomaterials is determined using circularly polarized transient absorption spectroscopy. The determination of the electron spin lifetime of the semiconductor colloidal nanomaterial involves placing the photochemically charged semiconductor colloidal nanomaterial on the sample holder of a transient absorption spectrometer, and performing transient absorption spectroscopy tests using circularly polarized excitation light and circularly polarized probe light. During the test, the kinetics decay rapidly when the circular polarization combinations of the excitation light and probe light are the same, while the kinetics are generated slowly when the circular polarization combinations of the excitation light and probe light are opposite. The electron spin relaxation lifetime is obtained by subtracting the kinetics of the excitation light and probe light in the same direction and opposite direction.

2. The method according to claim 1, characterized in that: The semiconductor colloidal nanomaterial is composed of semiconductor colloidal nanoparticles with light absorption capability. The semiconductor colloidal nanoparticles are placed in a toluene solution with a concentration of 0.1–1 mmol / L, and the absorbance in a 1 mm cuvette is 0.3–0.5 OD.

3. The method according to claim 1 or 2, characterized in that: The semiconductor colloidal nanomaterial is one or more of the following two types of substances; Category 1: A compound consisting of one or more elements from Group 2 and Group 5 of the periodic table, wherein the Group 2 element is one or more of zinc, cadmium, or mercury, and the Group 5 element is one or more of sulfur, selenium, or tellurium. The second category consists of trihalomethane perovskite colloidal nanomaterials with an ABX3 structure, wherein A = one or more of Cs, CH3NH3, or CH(NH2)2; B = one or more of Pb or Sn; and X = one or more of Cl, Br, or I.

4. The method according to claim 1, characterized in that: The morphological characteristics of the semiconductor colloidal nanomaterials are any one of semiconductor colloidal quantum dots, nanosheets, and nanorods; Semiconductor colloidal nanomaterials can be either homogeneous or have a core / shell heterostructure.

5. The method according to claim 1, characterized in that: The electron dopant is a tetrahydrofuran solution of strongly reducing lithium triethylborohydride (LiEt3BH) with a concentration of 0.001–0.02 mol / L.

6. The method according to claim 1, characterized in that: The photochemical charge doping steps are as follows: 1) Dilute 0.1 mL of 1 mol / L LiEt3BH tetrahydrofuran solution to 0.01–0.02 mol / L with 4.9–9.9 mL of toluene solution under a nitrogen atmosphere; 2) Stir a 0.5–2 mL toluene solution of 0.4 mmol / L semiconductor colloidal nanomaterials under a nitrogen atmosphere for 10–20 minutes; 3) Under light irradiation, use a syringe to drop 20–80 mL of diluted LiEt3BH tetrahydrofuran toluene solution into the above 0.5–2 mL toluene solution of semiconductor colloidal nanomaterials to carry out a chemical reaction. Irradiation time: 10–20 minutes.

7. The method according to claim 1 or 6, characterized in that: The photon energy of the light source under the specified illumination conditions is greater than the optical band gap of the semiconductor colloidal nanomaterial, where the band gap E is greater than the optical band gap of the semiconductor colloidal nanomaterial. g The range is 0 < E g < 5.0 eV.

8. The method according to claim 7, characterized in that: The photon energy of the light source under the specified illumination conditions is greater than the optical band gap of the semiconductor colloidal nanomaterial, where the band gap E is greater than the optical band gap of the semiconductor colloidal nanomaterial. g The range is 2.0–3.0 eV.

9. The method according to claim 1 or 7, characterized in that: The photon energy of the circularly polarized excitation light is equal to the optical band gap energy of the semiconductor colloidal nanomaterial, and the photon flux of the excitation light is controlled within 1–10. m J / cm 2 Within this range, the ratio of the excited transient signal to the band-edge exciton absorption intensity in the steady-state absorption spectrum is less than or equal to 1:10.

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