Particle spectrometry

Abstract

Particle spectrometer apparatus 1 for analysing particles emitted from, or transmitted through, a sample 10, the apparatus comprising: a moveable sample holder 8 comprising a coolable contact surface

Description

Field of the Invention The present invention relates to particle spectrometry. In particular, though not exclusively, the invention relates to X-ray photoelectron spectrometry, Ultraviolet photoelectron spectrometry, and electron spectrometry more generally (e.g., electron energy loss spectrometry (EELS) and related techniques, or Auger electron spectrometry), and Secondary Ion mass spectrometry, SIMS. Background The present invention has been devised in light of the above considerations. Particle spectrometry, such as X-ray photoelectron spectroscopy (XPS), Ultraviolet photoelectron spectrometry, Auger electron spectrometry, and Secondary Ion mass spectrometry (SIMS), as examples, are analytical techniques that each use irradiating particles (i.e., X-ray photons, UV photons, electrons, or primary ions, respectively) to irradiate a sample material under study, so as to cause particles in the form of electrons or secondary ions, to be emitted from a substance contained within the surface of the sample. Electron energy loss spectrometry (EELS) is an analytical technique that uses the energy distribution of incident electrons with a known, narrow range of kinetic energies, that are transmitted through a thin sample to analyse the content of the sample. Some of the electrons scatter inelastically within the sample. The amount of energy loss resulting from such scattering can be measured via an electron spectrometer to provide information about the sample. A spectrum of the kinetic energy of emitted, or transmitted, particles (i.e., electrons, photoelectrons, or secondary ions) is measured which describes the relative intensity of the emitted, or transmitted, particles as a function of their kinetic energy. The spectra produced in this way can be diagnostic of the sample understudy. A spectrum of the mass-to-charge ratio of emitted secondary ions is measured which describes the relative intensity of secondary ions as a function of their mass-to-charge ratio. In order to allow an unimpeded flow of particles (e.g., transmitted electrons, emitted photoelectrons or secondary ions) from the sample under study, the particle emission process must be performed in a high vacuum environment. To maintain a high vacuum environment in the sample analysis chamber it is important to connect to a “load-lock” chamber via an open and closable aperture, where the “load-lock” can be exposed to atmosphere for sample introduction and put to high vacuum quickly without breaking the vacuum environment of the sample analysis chamber. In addition, it is generally necessary to prepare certain sample categories such as biomaterials for particle spectrometry analysis by appropriately drying it and freezing it to a solid form. To facilitate this preparation step, many particle spectrometry devices provide a sample preparation chamber in which the sample drying and / or the sample freezing process may be performed. Such particle spectrometry devices also provide a separate sample analysis chamber in communication with the sample preparation chamber. Sample manipulation mechanisms of the particle spectrometry device are provided for moving a prepared sample from the sample preparation chamber to the sample analysis chamber for X-ray, UV, electron or primary ion irradiation and resultant emitted (or transmitted) particle detection. The freezing step of the preparation process is typically performed as follows. A sample is placed upon a sample holder e.g., in the form of a platform or plate comprised of a thermally conductive material, such as a Copper, upon the surface of which the sample is arranged. The sample plate is then placed within a sample preparation chamber of the particle spectrometry apparatus, which is typically a vacuum chamber. The sample preparation chamber is reduced to a vacuum state and the sample holder is placed in physical contact with a contact surface of a cooling device provided by the particle spectrometry device within the sample preparation chamber. The contact surface is typically formed form a thermally conductive material, such as a Copper, and the cooling device is configured to maintain the temperature of the contact surface at a very low temperature, such as below -100 degrees Celsius. The physical contact formed between the sample holder and the contact surface permits a flow of heat from the sample holder, and the sample upon it, into the contact surface of the cooling device. Given sufficient time, the flow of heat from the sample leads to a freezing of the sample. After freezing of the sample, the sample holder must then be transferred to the sample analysis chamber without the temperature rising to cause melting or sublimation of the sample. However, this freezing process is time-consuming and impedes the efficiency with which multiple samples may be prepared and analysed by a particle spectrometry device. Cooling inefficiency limits the size and shape of the sample holders that can be used, and can result in samples warming up above their melting or sublimation point during transfer from “load-lock” chamber to a sample analysis chamber. The present invention aims to address this limitation. Summary of the Invention In a first aspect, the invention may provide a spectrometer apparatus for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising: a vacuum chamber for receiving a sample for pre-analysis preparation and / or for analysis; a cooling unit comprising a cooling contact surface presented within the vacuum chamber, wherein the cooling unit is configured to cool the cooling contact surface to a temperature below an ambient temperature; a moveable sample holder for holding the sample and comprising a coolable contact surface for separably contacting the cooling contact surface; wherein at least one of the cooling contact surface and the coolable contact surface comprises a thermal interface material the surface roughness of which is less than the surface roughness of the other of the cooling contact surface and the coolable contact surface. A thermal interface material (TIM) may be a material that is disposed between two components in order to enhance the thermal coupling between them. The thermal interface material may be a solid material presenting a solid surface. The cooling unit may be arranged to cool the cooling contact surface to a temperature of about -100 degrees Celsius or less, or preferably about-150 degrees Celsius or less than about -170 degrees Celsius (e.g., about -196 degrees Celsius, or 77 Kelvin). By “surface roughness” it may be meant a profile roughness as defined according to the ISO 4287:1997 standard. This standard quantifies surface roughness in terms of a profile roughness parameter, Ra, proportional to the arithmetic average of surface profile height deviations from the mean deviation value determined from deviations along the evaluation length of the assessed surface profile. The profile roughness parameter, Ra, is typically expressed in terms of micrometres pm, or nanometres, nm. Reducing the surface roughness of at least one of the cooling contact surface and the coolable contact surface enhances mechanical contact by increasing the number of points of contact between the cooling contact surface and the coolable contact surface at a microscopic scale. In turn, this increases the number of heat flow pathways between the contacting surfaces. This efficient cooling of samples allows more effective and rapid cooling, and also permits the analysis of new samples that are otherwise incompatible with ultra-high vacuum environments (e.g., permitting cryogenic crystallisation). For example, the cooling unit may be arranged to perform cryogenic freezing of samples (e.g., biological samples, such as bacterial samples etc.) by rapidly crystallising liquids within the sample. This prevents the liquids from boiling off in the vacuum environment of the vacuum chamber once the pressure has reduced to vacuum levels there (e.g., by action of vacuum pump(s)). For example, at least one of the cooling contact surface and the coolable contact surface may have a surface roughness, Ra, which between about 0.6 pm and about 0.4 pm, or is less than about 0.5 pm or less than about 0.25 pm, or less than about 0.1 pm, or less than about 0.01 pm. The surface roughness of the other of the cooling contact surface and the coolable contact surface may have a surface roughness which is greater than about 0.5 pm, or greater than about 0.6 pm, or greater than about 1.0 pm, or greater than about 2.0 pm. The thermal interface material of the at least one of the cooling contact surface and the coolable contact surface may comprise the thermal conductance that is higher than the thermal conductance of the material of other of the cooling contact surface and the coolable contact surface. The thermal interface material may comprise diamond (e.g., a polycrystalline diamond layer, or a single crystal diamond layer) forming at least a part of one of the cooling contact surface and the coolable contact surface. For example, the thermal interface material may comprise a polycrystalline diamond layer which may be formed as a chemical-vapour deposited (CVD) diamond layer. Polycrystalline diamond can be produced in a variety of forms where growth conditions will influence the thermal conductivity, as is discussed in reference [1], Known methods of chemical-vapour deposition may be used to form the diamond layer having thermal properties such as are discussed in: [1] J.E. Graebner et al.: “Report on a second round robin measurement of the thermal conductivity of CVD diamond'; Diamond and Related Materials 7 (1998) 1589-1604. It is known in the art that polishing of polycrystalline diamond substrates may produce a surface roughness, Ra, of as little as 500 nm, as discussed in: [2] Chen Xiao et al.: “Polishing of polycrystalline diamond using synergies between chemical and mechanical inputs: A review of mechanisms and processes’’: Carbon, 196 (2022) 29-48. Alternatively, the thermal interface material may comprise single-crystal diamond. It is known in the art that mechanical polishing of a single crystal diamond substrates may produce a surface roughness, Ra, of as little as about 1.0 nm, or less, as discussed in: [3] Hu Luo et al. - “Polishing and planarization of single crystal diamonds: state-of-the-art and perspectives"; 2021 Int. J. Extrem. Manuf. 3 022003. Coupled with the very high thermal conductivity of polycrystalline diamond or single-crystal diamond, these features significantly enhance heat transfer rates. For example, at a temperature of 25 degrees Celsius, polycrystalline diamond has been shown to have a thermal conductivity of between about 1300 W / mK to about 2000 W / mK, and gem-quality natural diamond can exhibit thermal conductivities of about 2000 W / mK at that temperature, as is discussed in reference [1], However, at a temperature of-150 degrees Celsius, the thermal conductivity of polycrystalline diamond may approach 3,500 W / mK (see Fig. 1 in reference [1]), and the thermal conductivity of single-crystal diamond may exceed 5,000 W / mK. The hardness of the polycrystalline diamond or single-crystal diamond as compared to metals (e.g., Copper), combined with the ability to provide a low (e.g., very low) surface roughness as compared to metals (e.g., Copper) permits an interface with a softer metal material (e.g., Copper) allowing microscopic flattening deformation of microscopic surface salient features of the softer metal surface which reduces the surface roughness of the metal surface at the interface, and enhances thermal contact. The hardness of the thermal interface material forming one of the cooling contact surface and the coolable contact surface may be at least about 10 on the Mohs hardness scale. The hardness the material forming the other of the cooling contact surface and the coolable contact surface maybe between about 2.5 and about 3 on the Mohs hardness scale. A determination of hardness may be according to a surface scratch hardness according to the Mohs scale. Such a determination may be conducted as described in the international standard ISO 6769. The thermal interface material may comprise graphene forming at least a part of one of the cooling contact surface and the coolable contact surface. For example, graphene may possess a thermal conductivity of between about 4500 W / mK and about 5000 W / mK, such as about 5000 W / mK at a temperature of 300 K. Alternatively, or in addition, the thermal interface material may comprise any of the following materials forming at least a part of one of the cooling contact surface and the coolable contact surface: Boron arsenide [e.g., with a thermal conductivity of between about 1000 W / mK and about 1500 W / mK, such as about 1300 W / mK at 300 K] Cubic boron nitride [e.g., with a thermal conductivity of between about 700 W / mK and about 800 W / mK, such as about 740 W / mK at 300 K] Carbon nanotube [e.g., with a thermal conductivity of between about 3000 W / mK and about 3500 W / mK, such as about 3180 W / mK at 300 K] The coolable contact surface may comprise the thermal interface material, wherein the moveable sample holder may comprise a thermally conductive body portion (e.g., comprising Copper) to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. The cooling contact surface may comprise the thermal interface material, wherein the cooling unit may comprise a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. The Silver brazing material may be disposed between the respective thermally conductive body portion and the thermal interface material and may allow a joint to be formed between the two. It is postulated, though not asserted, that the braze fills the surface roughness of the two joined parts and thus gives a contiguous thermal path to / from the thermal interface material. By the term “Silver braze” or “Silver brazing" it may be meant a Silver alloy comprising another metal, such as one or more of: copper, zinc, nickel, manganese, cadmium, tin, silicon, etc. Silver brazing alloys typically contain from about 10% to about 90% Silver, alloyed with another metal. Commonly, a Silver braze alloy material may comprise 33% Silver and have a melting point of about 720°C, or may comprise 40% Silver and have a melting point of about 675°C, or may comprise 55% Silver and have a melting point of about 650°C. Brazing is a term of art known to refer to a metal-joining process whereby e.g., two metal work pieces are joined by flowing a molten filler metal into a joint between the two work pieces. The filler metal has a lower melting point than that of the metal of the work pieces. Brazing may be understood to differ from welding because it does not involve a melting of work pieces being joined. Brazing may be understood to differ from soldering because it involves higher temperatures and more closely fitted parts than achieved by soldering. The difference between soldering and brazing may be defined by the melting temperature of the filler metal. A solder material may be defined as a filler metal which has a melting temperature below 450°C. A brazing material may be defined as a filler metal with a melting temperature exceeding 450°C. The coolable contact surface may comprise copper, or an alloy thereof. The cooling contact surface may comprise copper, or an alloy thereof. In a second aspect, the invention may provide spectrometer apparatus for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising: a vacuum chamber for receiving a sample for pre-analysis preparation and / or for analysis; a cooling unit comprising a cooling contact surface presented within the vacuum chamber, wherein the cooling unit is configured to cool the cooling contact surface to a temperature below an ambient temperature; a moveable sample holder for holding the sample and comprising a coolable contact surface for separably contacting the cooling contact surface; wherein at least a part of each of the cooling contact surface and the coolable contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube. The moveable sample holder may comprise a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. The thermal interface material may comprise a thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion of the moveable sample holder. The cooling unit may comprise a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. The thermal interface material may comprise a thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion of the cooling unit. In a third aspect, the invention may comprise a sample holder for holding a sample within a vacuum chamber of a spectrometer for pre-analysis preparation and / or for analysis of particles emitted from, or transmitted through, the sample , the comprising a cooling unit comprising a cooling contact surface presented within the vacuum chamber, the cooling unit being configured to cool the cooling contact surface to a temperature below an ambient temperature, wherein: the sample holder comprises a coolable contact surface for separably contacting the cooling contact surface wherein the coolable contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube forming at least a part of the coolable contact surface. For example, the vacuum chamber of the spectrometer for pre-analysis preparation may be used as a “load-lock” to allow sample introduction without breaking vacuum in the analysis vacuum chamber for analysis of particles. The moveable sample holder may comprise a thermally conductive body portion (e.g., comprising copper, or an alloy thereof) to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. In a fourth aspect, the invention may provide a spectrometer apparatus for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising: a vacuum chamber for receiving the sample holder upon a moveable sample holder for pre-analysis preparation and / or for analysis; a cooling unit comprising a cooling contact surface presented within the vacuum chamber for forming a separable contact with a coolable contact surface of the sample holder; wherein the cooling unit is configured to cool the coolable contact surface to a temperature below an ambient temperature and the cooling contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube forming at least a part of one of the cooling contact surface. The cooling unit may comprise a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material. The invention includes the combination of the aspects and preferred features described except where such a combination is clearly impermissible or expressly avoided. According to any aspect disclosed herein, the material of the sample holder body may comprise Copper or Molybdenum. According to any aspect disclosed herein, the material of the cooling contact surface may comprise stainless steel. According to any aspect disclosed herein, the thickness of the thermal interface material may be between about 350 / zm and about 450 / zm, such as about 400 / zm (e.g., for poly-crystalline diamond etc.). According to any aspect disclosed herein, the thickness of the joining material (e.g., Silver braze) may be between about 50 / zm and about 150 / zm, such as about 100 / zm. The emitted, or transmitted, particles may comprise Auger electrons in an Auger Electron Spectrometer or scattered primary electrons as in an Electron Energy Loss spectrometer, EELS. The spectrometer according to any aspect disclosed herein, may comprise an electron spectrometer. The spectrometer according to any aspect disclosed herein, may comprise an X-ray photoelectron spectrometer, XPS, for illuminating the sample with X-rays therewith to generate an emission of photoelectrons from the sample for analysis. The spectrometer according to any aspect disclosed herein, may comprise an Ultraviolet photoelectron spectrometer for illuminating the sample with ultraviolet photons therewith to generate an emission of photoelectrons from the sample for analysis. The spectrometer according to any aspect disclosed herein, may comprise an Auger electron spectrometer for irradiating the sample with electrons therewith to generate an emission of electrons from the sample for analysis. The spectrometer according to any aspect disclosed herein, may comprise a Secondary Ion mass spectrometer, SIMS, for irradiating the sample with ions therewith to generate an emission of secondary ions from the sample for analysis. The spectrometer according to any aspect disclosed herein, may comprise an electron energy loss spectrometer, EELS. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 schematically shows an X-ray photoelectron spectrometer, XPS, in sample analysis mode. Figure 2 schematically shows the X-ray photoelectron spectrometer, XPS, of Fig. 1 prior to sample analysis mode, and in sample preparation mode. Figure 3 schematically shows a cooling unit and sample holder of the X-ray photoelectron spectrometer, XPS, of Fig.1. Figure 4 schematically shows contacting surface parts a cooling unit and sample holder of an X-ray photoelectron spectrometer, XPS, according to the prior art. Figure 5 schematically shows the contacting surface parts of Fig.4 when subject to mechanical compression. Figure 6 schematically shows a heat-flow diagram of two contacting surface parts according to the prior art. Figure 7 schematically shows a heat-flow diagram of two contacting surface parts in accordance with the invention comprising a thermal interface material. Figure 8 schematically shows a cooling unit and sample holder of an X-ray photoelectron spectrometer, XPS, according to the invention. Figure 9 schematically shows a cooling unit and sample holder of an X-ray photoelectron spectrometer, XPS, according to the invention. Figure 10 schematically shows a cooling unit and sample holder of an X-ray photoelectron spectrometer, XPS, according to the invention. Figure 11 schematically shows the X-ray photoelectron spectrometer, XPS, prior to sample transfer for either sample analysis or sample preparation by cooling. Figure 12 schematically shows a cooling unit of an X-ray photoelectron spectrometer, XPS, according to an example of the invention according to Figure 10. Figure 13 schematically shows a cold finger of a cooling unit of an X-ray photoelectron spectrometer, XPS, according to an example of the invention comprising a cooling unit according to Figure 10. Figure 14 schematically shows a cold finger of a cooling unit of an X-ray photoelectron spectrometer, XPS, with a sample holder attached to it, according to an example of the invention comprising a cooling unit and sample holder according to Figure 10. Figures 15A to 15K schematically show a sequence of operations of a sample transfer apparatus in an X-ray photoelectron spectrometer. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. For the avoidance of doubt, it is to be understood that the present invention relates to particle spectrometry in general. In particular, though not exclusively, the invention relates to X-ray photoelectron spectrometry, Ultraviolet photoelectron spectrometry, and electron spectrometry more generally (e.g., electron energy loss spectrometry (EELS) and related techniques, or Auger electron spectrometry), and Secondary Ion mass spectrometry, SIMS. To provide a better understanding of the invention, the following disclosures describe examples of the invention in terms of X-ray photoelectron spectrometry (XPS), however, it should be understood that the choice of X-ray photoelectron spectrometry as an example is purely for convenience and is not intended to be limiting. Referring to Figure 1 and Figure 2, Figure 1 shows a schematic illustration of an X-ray photoelectron spectrometer 1, XPS, according to the invention, in analysis mode, whereas Figure 2 shows the XPS apparatus in sample preparation mode. The XPS device 1 comprises a sample preparation chamber 2 in which the freezing of a sample 10 may be performed. The sample preparation chamber 2 is separate from, but able to be in communication with, a sample analysis chamber 4 via a communication port that is openably closed by a moveable door 6 which is moveable between a closed position (6) and an open position (6a) as shown by the dashed double-headed arrow 150. A sample manipulation mechanism (12, 14) is configured for moving a prepared sample 10, when the moveable door 6 is in an open position, 6a, from the sample preparation chamber 2 to the sample analysis chamber 4 for X-ray irradiation and photoelectron detection. A first vacuum pump 30 is configured in communication with the sample analysis chamber and is arranged to produce the desired vacuum state within the sample analysis chamber as appropriate for XPS sample analysis to take place. A second vacuum pump 32 is configured in communication with the sample preparation chamber and is arranged to produce the desired vacuum state within the sample analysis chamber as appropriate for XPS sample preparation to take place. The XPS device 1 comprises a sample holder 8 upon an upper surface of which the sample 10 is arranged. The sample holder presents a coolable contact surface 9 which is arranged for forming a physical contact with a cooling surface 38 of a cooling unit of the XPS apparatus, as is discussed in more detail below. The sample holder 8 is attached to a distal end of a sample manipulator arm 12 which is in turn connected to a manipulation motor, or actuator, 14 configured to apply an urging force to the manipulator arm 12 to extend or retract the manipulator arm linearly in a direction away from or towards (see dashed double-headed arrow 125) the manipulator motor. As a result, the position of the sample plate 8 may be manipulated. In Figure 1, the sample manipulator arm 12 has placed the sample 10 in a position for sample analysis to take place. With the moveable door 6 in a position (6a) to open the communication port between the sample analysis chamber, the sample manipulator arm 12 is able to be retract (Figure 2) or extended (Figure 1) to move the sample plate 8 into the sample preparation chamber 2 or into the sample analysis chamber, respectively. With the moveable door 6 in a position to close the communication port between the sample analysis chamber and the sample preparation chamber, the first vacuum pump is operated to produce the desired vacuum state within the sample analysis chamber. An X-ray source 16 is provided within the sample analysis chamber and is configured to irradiate the sample 10 with X-rays 18 thereby to generate an emission of photoelectrons 20 from the surface of the sample 10. It is to be understood that in this schematic example, X-rays are shown as directly irradiating the sample, but in other examples (not shown) the X-rays may be reflected onto the sample, for example by using an X-ray gun arranged to fire non-monochromatic X-rays at a monochromating mirror which reflects the incident X-rays in a direction onto the sample. Alternatively, as shown in Figure 1, the X-ray source may be arranged to generate achromatic X-rays. An electron lens assembly 22 guides emitted photoelectrons into an electron analyser assembly 24 comprising an electron optical assembly configured to discriminate the trajectories 26 of photoelectrons according to their kinetic energy. A photoelectron detector 28 is configured to receive the photoelectrons at spatial positions upon the detectorthat are determined according to the discriminated trajectories 26 of the photoelectrons. With this detection information, a photoelectron spectrum may be generated. Typical operating conditions require a pressure of e.g., less than 10-8 Torr within the sample analysis chamber. This is required because the emitted photoelectrons have a relatively low energy and may readily interact with any gas molecules which they encounter (e.g., scatter or are readily absorbed by ambient atmosphere). The electron energy analyser 24 used to discriminate among the energies of the photoelectrons that are produced may be a Hemispherical Analyzer (HSA), or the like. In Figure 2, the sample manipulator arm 12 has retracted and placed the sample 10 in a position for sample preparation to take place. A cooling unit (40, 34) of the XPS device comprises a coolant source 40 providing a supply of liquid nitrogen 36 which is controlled to flow along an internal flow duct within an elongate cooling portion 34 (sometimes referred to in the art as a “cold finger”) to a distal end of the internal flow duct terminated by a distal closure end wall. Figure 3 schematically shows a cooling unit and sample holder of the X-ray photoelectron spectrometer of Figure 2, in which like items are assigned like reference symbols as between Figure 2 and Figure 3. As shown schematically in Figure 3, the provision of liquid nitrogen may be achieved by inserting gaseous nitrogen into the internal flow duct that is cooled by a liquid nitrogen reservoir where it becomes a liquid. It may be that some of the inserted gaseous nitrogen does not liquify along the whole of the internal flow duct, however, sufficient liquification is achievable for successful cooling of the cold finger. Alternatively, liquid nitrogen may be inserted into the internal flow duct. A baffle within the flow duct provides a separation of the input coolant and the output / returned coolant. The temperature of the closure wall is maintained in thermal equilibrium with the temperature of the liquid nitrogen so as to maintain a very low temperature (e.g., -196 degrees Celsius, or 77 Kelvin). This sample preparation chamber is the same area in which samples are loaded into the instrument. This allows a sample to be placed in the sample preparation chamber in ambient atmospheric pressure conditions, and subsequently reduce the chamber pressure to ultra-high vacuum (UHV) pressures by pumping down the pressure within the sample preparation chamber. At the same time, the sample holder 8 may be rapidly cooled whilst in thermal and mechanical contact with the cooling unit (40, 34). This allows the user to perform cryogenic freezing of samples (e.g., biological samples, such as bacterial samples etc.) so as to rapidly crystallise liquids within the sample to stop them from subliming or boiling off in the UHV environment once the pressure has reduced to UHV levels. The outer surface 38 of the closure end wall of the internal flow duct presents a flat surface defining a cooling contact surface. This cooling contact surface is configured to form a physical contact with a coolable contact surface 9 of the sample plate. As shown in Figure 2, the manipulation motor, or actuator, 14 is configured to retract the manipulator arm 12 into the sample preparation chamber to an extent that brings the coolable contact surface 9 of the sample plate 8 into physical contact with the cooling contact surface 38 of the cooling unit. The manipulation motor, or actuator, may maintain a force via the manipulator arm 12 that has the effect of urging the two contact surfaces (9, 38) together thereby applying a mechanical compression force between the two contact surfaces. In other embodiments, a mechanical gripper attached to the end of the elongate cooling portion 34 may engage and grip the sample plate 8 when the elongate cooling portion 34 and the sample plate 8 are in contact so as to provide a mechanical compression force to urge the two together at their contact surfaces. The freezing step of the preparation process is typically performed as follows. A sample 10 is placed upon a sample holders e.g., in the form of a platform or plate comprised of a thermally conductive material, such as a Copper, upon the surface of which the sample is arranged. The sample holder 8 is then placed within a sample preparation chamber 2 of the XPS apparatus by attachment to the end of the manipulator arm 12, as shown in Figure 2. The sample manipulation chamber contains a cold finger 34 and in some embodiments this may be moveable to place it in physical contact with the sample holder inside the sample preparation chamber, or on some embodiments the sample holder may be moved into physical contact with the cold finger 34 as indicated in Figure 2. In some embodiments, the sample holder 8 may comprise a cold finger. The sample preparation chamber 2 is then reduced to a vacuum state while the sample holder 8 is placed in physical contact with a contact surface 38 of a cooling unit (40, 34) within the sample preparation chamber, as described above. The contact surface is typically formed form a thermally conductive material, such as a Copper, and the cooling device (40, 34) is configured to maintain the temperature of the contact surface at a very low temperature, such as -196 degrees Celsius. The physical contact formed between the sample holder and the contact surface permits a flow of heat from the sample holder, and the sample upon it, into the contact surface of the cooling device. Given sufficient time, the flow of heat from the sample leads to a freezing of the sample. Once frozen, the sample holder 8, and the sample 10 upon it, may be moved into the sample analysis chamber 4, by extension of the manipulator arm 12, for XPS analysis as described above with reference to Figure 1. The inventors have realised that the process of freezing the sample in this way may be greatly improved upon by the use of a thermal interface material to form at least a part of one or both of the cooling contact surface 38 and the coolable contact surface 9. This realisation has come from an appreciation of the physical and thermal processes occurring at a microscopic level when the cooling contact surface and the coolable contact surface come into physical contact. In the absence of any such thermal interface material, the cooling contact surface of the cooling unit and / or the coolable contact surface of the sample holder may appear to be smooth on a macroscopic scale but is typically rather rough on a microscopic scale. On a microscopic scale, such a surface lacking any thermal interface material is composed of asperities (i.e., local ‘peaks’) separated by interstices (i.e., local ‘valleys’). Consider two such surfaces, both lacking any thermal interface material and each placed in contact with the other. Figure 4 schematically illustrates this arangement, in which opposing contact surfaces present asperities and interstices to each other. Physical contact between these opposing surfaces is achieved only when two opposing asperities are able to make contact, or when an asperity of one surface is able to reach contact with an opposing interstice. An increase in the number of such contact points can be achieved by pushing opposing asperities in to opposing interstices to create new contacts, as shown schematically in Figure 5. In more detail, Figure 4 shows a schematic view of parts of two opposing contact surfaces each lacking any thermal interface material. A first contact surface 100 comprises interstices 42 between surface asperities 44. A second contact surface 200 comprises interstices 42 between surface asperities 44. The mean surface level of the surface of the first contact surface is separated from the mean level of the surface of the second contact surface by a separation dl. In non-vacuum conditions, an interstitial substance (e.g., air) separates most parts of the first contact surface from forming a physical contact with most parts of the second contact surface in the absence of mechanical pressure applied to urge the two contact surfaces together. Some heat flow through this air is possible to permit heat transfer from one contact surface to the other. In vacuum conditions, however, the only thermal pathways available are via contact points between the two contact surfaces. Only a relatively small number of physical points of contact 46 exist between the first contact surface and the second contact surface thereby providing only a few heat flow pathways 48 in the absence of applied mechanical pressure. The effect of applying such mechanical pressure is schematically illustrated in Figure 5. The effect is to cause surface asperities of at least one of the first and second contact surfaces to cause additional contact points to be formed between additional asperities and opposing interstices. This causes asperities of at least one contact surface to make additional thermal contacts (see contact points 48, Fig. 5) with the other heating surface. Additional heat flow pathways 48 are formed between the opposing contact surfaces to allow the transfer of extra heat between the opposing surfaces. In particular, Figure 4 shows a situation in which there exists an initial separation, dl, between mean surface level of the first contact surface 100 and the mean surface level of the second contact surface 200. At this separation, only a few of the surface asperities 44 of either one contact surfaces makes contact with surface asperities 44 of the other. However, as shown in Figure 5, by pushing the two surfaces closer together, a new, lesser separation, d2, is achieved between mean surface levels of the first contact surface and the second contact surface. At this lesser separation many more, but not all, of the surface asperities 44 of each contact surface make contact with surface asperities 44 of the other. Reducing the separation, d2, even further, by pressing the two contact surfaces together with increased pressure, causes surface asperities already in contact to compress and deform plastically thereby to allow lesser asperities on opposing surfaces to form yet more new contacts. This increases the number and density of thermal flow pathways between the two opposing contact surfaces. Consequently, where the opposing contact surfaces are made of a or other thermally conductive material (e.g., a metal) an increase the number of heat flow pathways 48 between the bare conductive surfaces of the opposing contact surfaces may arise due to the applied pressure urging the conductive face of one contact surface into increased contact with bare conductive opposing parts of the opposing contact surface. This provides an increased area of direct contact between parts of the face of one such contact surface 100 and opposing parts of the face of the other contact surface 200 with reference to Figure 4 and Figure 5. Despite the increase in the number of heat flow pathways achieved by mechanically pressing the two opposing contact surfaces together, the result remains that many spaces or voids exist between opposing surface parts and the number of physical contact points accounts for only a very small proportion of the surface area available upon either contact surface at which no contact point is formed. Put in other words, the thermal coupling between the two contact surfaces is poor. This means that when considered in the present context, whereby one of the two contact surfaces (100, 200) is a cooling contact surface of a cooling unit in an XPS apparatus, and the other of the two contact surfaces is a coolable contact surface of a sample holder, the cooling efficiency of the cooling contact surface when in contact with the coolable contact surface, is poor. Referring to Figure 6, a simple thermal model in the form of a “composite wall” heat transfer model, permits one to quantify the heat transfer between two such contact surfaces using Fourier’s equation: dT Q — kA ~— dx Here, Q is the heat flow through the body in question, k is the thermal conductivity of a material forming part of the body, A is the cross-sectional area of the surface of contact / boundary between two parts of the body, and dT / dx is the temperature gradient in the direction of heat flow. A simple composite plane wall made of two materials is shown in Figure 6. The two materials have, respectively, thermal conductivities k± and k2 (W / m.K), thicknesses XI and X2 (metres, m) and are in direct contact with each other at a boundary of area A (metres squared, m2) and temperature T2 (Kelvin, K) across which a heat flux Q (Watts, W) flows according to: k^A Q = -^(t2-t^ Here, the material of thermal conductivity kt presents an outer boundary at a thermal input side of the wall at a higher temperature Ti (i.e., Tj. >T2). Similarly, the material of thermal conductivity k2 presents an outer boundary at a thermal output side of the wall at a lower temperature T3 (i.e., T2 >T3), such that: Q = A2 Conditions of one-dimensional steady state conduction are shown. Thermal contact conductance (W / m2.K) quantifies heat conduction between the two solid bodies in thermal contact. The thermal contact conductance coefficient, hc, is a property indicating the thermal conductivity between the two bodies in contact. The inverse of the thermal contact conductance coefficient is known as the thermal contact resistance, TCR = l / hc. The two solid bodies of this model are pressed together at their interface and knowledge of the thermal performance of the contact interface is significant in terms of the temperature drop (T2) across the interface between the two bodies. TCR significantly depends both on the surface roughness of the contact surfaces of each of the two bodies where they form an interface, and on the force or pressure applied to the two surfaces to urge them together as discussed above with reference to figures 4 and 5. As noted above, as the two such contact surfaces are pressed together, asperities (‘peaks’) may form effective new points of contact and thermal flow pathways, whereas the interstices (voids) between asperities do not. These interstices, when under vacuum conditions, act as thermal insulation at the contact interface and therefore the effective contact area between the two bodies at the contact interface is dramatically smaller than the apparent contact area. In the case of metallic composite material, which is placed in a vacuum, thermal conduction through the contact spots is the primary mode of heat transfer. Referring to Figure 7, according to an embodiment of the invention, the inventors have discovered that an effective way to reduce thermal contact resistance, TCR, at a contact Interface between a cooling contact surface of an XPS cooling unit, and a coolable contact surface of a sample holder of the XPS device, is to utilise a thermal interface material (TIM). The inventors have found that poly-crystalline diamond (or alternatives such disclosed herein, including single-crystal diamond or graphene etc.) is found to dramatically improve the cooling performance of the XPS device by reducing the TCR due to a number of key factors. Without wishing to be bound by theory, the inventors propose the following explanation of the benefits of using poly-crystalline diamond as a thermal interface material to form a surface of one or both of the cooling contact surface of an XPS cooling unit, and a coolable contact surface of a sample holder of the XPS device. One such factor is that poly-crystalline diamond has, or can be provided with, a very low surface roughness. This can be expected to mean that if this thermal interface material is used both as the cooling contact surface of an XPS cooling unit, and as coolable contact surface of a sample holder of the XPS device, then a far greater proportion of the two surfaces will make physical contact points at a microscopic level, thereby increasing the number of heat flow pathways available. Furthermore, if this thermal interface material is used only as the cooling contact surface of the XPS cooling unit, or only as the coolable contact surface of a sample holder of the XPS device, then the high hardness of the polycrystalline diamond can be expected to be more effective at deforming the asperities of the opposing contact surface (e.g., metallic, non-diamond) when the two surfaces are pressed together. A deformation of the asperities of the softer, non-diamond contact surface can be expected to be a flattening of those asperities so as to some extent to ‘spread’ the initially narrow asperity peaks across the harder, flatter poly-crystalline diamond surface thereby increasing the effective surface area of the contact interface in ways not achievable otherwise. In addition, the high thermal conductance of poly-crystalline diamond assists in enhancing the rate of flow of heat through the contact interface between the two contact surfaces. Silver braze material is used to bond the cold finger and the poly-crystalline diamond whereby “surface wetting” of the braze material at least partially fills asperities between the cold finger and the poly-crystalline diamond surface. Figure 7 schematically shows the nature of the enhancement in heat flow achievable. Performance as compared with the conventional cooling arrangements, (i.e., without TIM) show that significant gains are possible. As noted above, graphene may be used as the thermal contact interface material in place of diamond. It is noted that graphene is very malleable or compliant and has a high thermal conductivity. The measured thermal conductivity of graphene is in the range 3000 - 5000 W / m.K at room temperature, an exceptional figure. This can be expected to mean that if this thermal interface material is used both as the cooling contact surface of an XPS cooling unit, and as the coolable contact surface of a sample holder of the XPS device, then the high compliance between the deformable surface asperities and interstices of the opposing surfaces, when pressed together, would result in a far greater proportion of the two surfaces making physical contact points at a microscopic level, thereby increasing the number of heat flow pathways available. Furthermore, if this thermal interface material is used only as the cooling contact surface of the XPS cooling unit, or only as the coolable contact surface of a sample holder of the XPS device, then the high malleability or compliance of the graphene can be expected to be more effective at deforming around (i.e., being penetrated by) the asperities of the opposing contact surface (e.g., metallic, non-diamond) when the two surfaces are pressed together. In addition, a compliant deformation of the softer graphene contact surface can be expected to include a ‘filling in’ of the interstices of the harder non-graphene contact surface thereby increasing the effective surface are of the contact interface in ways not achievable otherwise. In addition, the high thermal conductance of graphene assists in enhancing the rate of flow of heat through the contact interface between the two contact surfaces. Figures 8, 9 and 10 show examples of embodiments according to the invention. For example, Figure 8 schematically shows parts of an X-ray photoelectron spectrometer 1, XPS, as shown in Figure 1 and Figure 2 for illuminating a sample with X-rays therewith to generate an emission of photoelectrons from the sample for analysis. As noted above, with reference to Figure 1 and Figure 2, the apparatus comprises a vacuum chamber (2, 4) for receiving a sample 10 for pre-analysis preparation (chamber part 2) and / or for analysis (chamber part 4). A cooling unit (34, 36, 38, 40) comprises a cooling contact surface 38 presented within the vacuum chamber configured to cool the cooling contact surface 38 to a temperature below an ambient temperature, such as below -150 degrees Celsius. A moveable sample holder 8 holds a sample 10. The sample holder comprises a coolable contact surface 9 for separably contacting the cooling contact surface 38. Both the cooling contact surface and the coolable contact surface each comprises a thermal interface material comprising poly-crystalline diamond and / or graphene (50A, 50B). The moveable sample holder 8 comprises a thermally conductive body portion 80 to a surface 9B of which the thermal interface material is joined by a joining material 54 comprising a Silver brazing material. Regarding the sample holder 8, the thermal interface material 50B upon it has a thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion 80 of the sample holder. Similarly, the cooling unit (34, 36, 38, 40) also comprises a thermally conductive body portion 340 to a surface 38A of which the thermal interface material is joined by a joining material 52 comprising a Silver brazing material. The Silver brazing material typically has a thermal conductivity which is greater than Copper (e.g., 406 W / m.K for a Silver brazing material as opposed to 385 W / m.K for Copper) such that the braze also helps with the thermal matching. Regarding the cooling unit 43, the thermal interface material has a thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion of the cooling unit. The cooling contact surface 38 and the coolable contact surface 9, in use, are pressed together by an urging force 58 provided by the manipulator arm 12 which urges the coolable contact surface against the cooling contact surface. Figure 9 schematically illustrates a variant of this embodiment in which the thermal interface material is not present upon the thermally conductive body portion 340 of the cooling unit 34, and also no joining material 52 (a Silver brazing material) is present, thereby leaving bare the thermally conductive body portion at the cooling contact surface 38. The urging force 58 provided by the manipulator arm 12 urges the thermal interface material 50B of the coolable contact surface 9 against the bare surface of the thermally conductive body portion at the cooling contact surface 38. In doing so, surface asperities 44 of the cooling contact surface are either ‘squashed flatter’ to some extent when the thermal interface material 50B is poly-crystalline diamond, or compliantly ‘deformed around’ when the thermal interface material 50B is graphene. Figure 10 schematically illustrates a further variant of this embodiment in which the thermal interface material is not present upon the thermally conductive body portion 80 of the sample holder 8, and also no joining material 52 (a Silver brazing material) is present, thereby leaving bare the thermally conductive body portion at the coolable contact surface 9. The urging force 58 provided by the manipulator arm 12 urges the bare surface of the thermally conductive body portion 80 of the sample plate, forming the coolable contact surface 9, against the thermal interface material 50A of the coolable contact surface 38. In doing so, surface asperities 44 of the coolable contact surface are either ‘squashed flatter’ to some extent when the thermal interface material 50A is poly-crystalline diamond, or compliantly ‘deformed around’ when the thermal interface material 50A is graphene. In other embodiments, such as schematically illustrated in Figure 11, the cold finger 34 of the cooling device (40, 34) is retractably extendable within the sample preparation chamber 2 to move the cooling contact surface 38 of the cooling device in a direction (see dashed double-headed arrow 100) towards or away from the sample holder 8 when the sample holder is positioned at a waiting position within the sample preparation chamber 2. Figure 12 shows an example of a retractably extendable cooling device (40, 34) in the retracted position. A manually operable actuator 400 comprises a manually operable turning wheel 401 coupled to a gearing assembly 410 (e.g., a worm gear assembly) configured to convert a turning motion of the turning wheel 400 into a linear motion (extension / retraction) of an actuator arm 411. The gearing assembly 410 is attached by a coupling part 403 to a static portion 40 of the cooling device (40, 34) comprising liquid coolant (e.g., liquid N2) flow lines 407 for providing a supply of liquid coolant to the cold finger 34. The distal end of the actuator arm 411 is attached by a coupling part 404 to the moveable cold finger 34 of the cooling device which includes, at its distal end, a cooling contact surface 38 formed by a film of poly-crystalline diamond 50A bonded to the metallic body of the cold finger 34 by Silver brazing material in the manner disclosed herein with reference to Figure 10. The static portion 40 of the cooling device is coupled to the moveable cold finger 34 via a cylindrical bellows 405 able to extend and / or compress in response to extension and retraction of the cooling finger. The liquid coolant flow lines 407 pass axially along the inner bore of the cylindrical baffle between the static portion 40 of the cooling device and the moveable cold finger 34. Telescopic guide rods 402 couple the gearing assembly 410 to the distal end of the actuator arm 411 via the coupling part 404 so as to support the actuator arm and cold finger, especially when in the extended state. The sample holder may be positioned at a waiting position within the sample preparation chamber 2 directly / manually by the user or remotely by use of the manipulator 12. The waiting position may be a preset location or position within the sample preparation chamber pre-aligned with a position or location of the cooling contact surface 38 of the cooling device achievable by extension of the cold finger 34. The manipulator may position the sample holder 8 at the pre-set location or position while the sample holder is attached to the manipulator and may then keep the sample holder 8 at the pre-set position for cooling by the cold finger 34, and subsequently (once cooled) may move the sample holder into the sample analysis chamber 4 for X-ray irradiation and photoelectron detection. Alternatively, the manipulator may deliver the sample holder 8 to a separate sample holder mounting assembly (not shown) within the sample preparation chamber 2 arranged to receive and hold the sample holder 8 at the pre-set position for cooling by the cold finger 34, and subsequent collection (once cooled) by the manipulator for delivery into the sample analysis chamber 4 for X-ray irradiation and photoelectron detection. In this way the cooling contact surface 38 may be deployed, as desired, to reversibly place it in physical contact with the coolable contact surface 9 of the sample holder 8, before the sample holder is positioned within the sample analysis chamber 4 for X-ray irradiation and photoelectron detection. In some embodiments, such as illustrated in Figure 13 and Figure 14 for example, the distal end of the cold finger 34 may comprise an urging assembly 500 configured to apply to the sample holder 8 an urging force, F, which urges the coolable contact surface 9 of the sample holder 8 against cooling contact surface 38 of the cold finger. The urging assembly comprises a clamp mechanism formed by a pair of clamp arms 500 each disposed at opposite lateral sides of the distal end of the cold finger 34 each being adjacent to a respective one of two opposite lateral sides of the cooling contact surface 38 of the cold finger 34. Each clamp arm comprises a longitudinal part extending in a direction parallel with the longitudinal axis of the cold finger and terminated by a transverse part extending in a direction transverse to the longitudinal axis so as to partially extend across a distal end surface of the cold finger adjacent to the cooling contact surface 38. The longitudinal part of each clamp arm is slidingly mounted to the distal end of the cold finger 34 so as to be slidingly moveable in a direction parallel to the longitudinal axis thereby to reversibly separate, in an axial direction, the transverse part from the distal end surface parts of the cold finger across which it extends. Each clamp arm is coupled to a spring (not shown) attached to the cold finger and arranged to apply a force to the respective clamp arm in a direction to urge the transverse part towards, and against, the distal end surface parts of the cold finger across which it extends. Two separate and mutually parallel mounting rails 502 extend from the distal end of the cold finger from a respective location between the transverse part of an adjacent clamp arm 500 and the cooling contact surface 38 of the cold finger, thereby each providing a mounting rail at either side of the cooling contact surface. Each mounting rail extends in a direction parallel with the longitudinal axis of the cold finger and the pair of mounting rails are configured to be simultaneously inserted into reciprocating mounting bores (not shown) formed at either side of the coolable contact surface 9 of the sample holder, so as to mount the sample holder to the distal end of the cold finger 34, as shown in Figure 14. The sample holder 8 comprises, or has attached to it, a pair of mounting ledges 504 over each respective one of which the transverse part of a respective one of the two clamp arms is able to be positioned to simultaneously clamp the mounting ledge in question to the distal end of the cold finger 34 by applying the urging force, F, to a respective clamped mounting ledge. This urges the coolable contact surface 9 of the sample holder 8 against cooling contact surface 38 of the cold finger with a mechanical compression force, F. The thermal interface material 50A of the cooling contact surface 38, e.g., comprising polycrystalline diamond and / or graphene, is thereby urged against the coolable contact surface 9 of the sample holder 8 to enhance thermal coupling therebetween. Figures 15A to 15K schematically show a sequence of operations of a sample transfer apparatus in an X-ray photoelectron spectrometer according to an example of the invention. The sample transfer apparatus comprises a first vacuum chamber 508 for receiving a sample for preanalysis preparation, and a second vacuum chamber 506 for sample analysis. The first vacuum chamber 508 of the spectrometer for pre-analysis preparation is separate from, but able to be in communication with, a sample analysis chamber 506 via a communication port that is openably closed by a moveable door 510 which is moveable between a closed position (Fig.15A) and an open position (Fig.15B). The first vacuum chamber 508 is used as a “load-lock” to allow sample introduction into the first vacuum chamber when the moveable door 510 is closed, thereby doing so without breaking vacuum in the second (analysis) vacuum chamber 506. A first cooling unit (34a, 40a) is disposed in the first vacuum chamber, and a second cooling unit (34b, 40b) is disposed in the second vacuum chamber. Each cooling unit is as disclosed herein with reference to figures 3 to 10 and 12 to 14 and comprises a cooling contact surface as disclosed herein. Each cooling contact surface is presented within the respective first or second vacuum chamber and each of the two the cooling units is configured to cool its respective cooling contact surface to a temperature below an ambient temperature. A sample manipulator arm 12 and associated manipulation motor, or actuator, 14 such as is disclosed herein with reference to figures 1,2 and 11 to 14, is provided within the second vacuum chamber 506. A distal end of the manipulator arm comprises an aforesaid urging assembly comprising the aforesaid clamp mechanism formed by a pair ofclamp arms 500. The sample manipulator arm 12 is able to extend or retract linearly in a direction away from or towards the manipulator motor. As a result, the position of a moveable sample holder plate 8 within the second vacuum chamber may be manipulated when attached to the manipulator arm via the clamp arms, as discussed below. The moveable sample holder plate 8, bearing a sample 10, presents a coolable contact surface 9 which is arranged for forming a physical contact with a cooling surface 38 of a cooling unit of the XPS apparatus, as is discussed in more detail below. The moveable sample holder as disclosed herein with reference to figures 12, 13 and 14 is provided in each one of the first and second vacuum chambers for holding the sample via a respective urging assembly comprising the aforesaid clamp mechanism formed by a pair of clamp arms 500. In particular, the moveable sample holder 8 is configured for holding a sample 10 and comprises an aforesaid coolable contact surface for separably contacting the cooling contact surface of the first cooling unit (34a, 40a) when disposed within the first vacuum chamber 508 during preparation before analysis, and for separably contacting the cooling contact surface of the second cooling unit (34b, 40b) when disposed within the second vacuum chamber 506 during subsequent XPS analysis. At least one of the cooling contact surface and the coolable contact surface comprises an aforesaid thermal interface material, as disclosed herein. A transport rail 512 extends linearly from the first vacuum chamber to the second vacuum chamber via the moveable door 510 when open. A delivery mechanism is disposed within the first vacuum chamber and carries the moveable sample holder 8 upon it and is configured to move back and forth along the transport rail from the first vacuum chamber to the second vacuum chamber for delivery of the second vacuum chamber and to return to the first vacuum chamber without the sample holder once delivered for XPS analysis. Similarly, the delivery mechanism is configured to return into the second vacuum chamber subsequently to collect the sample holder after XPS analysis is completed, and to return to the first vacuum chamber with the collected sample holder. Figures 15A to 15K schematically show a sequence of operations of a sample transfer apparatus as follows: Figure 15A: Initially, the first vacuum chamber contains the moveable sample holder 8 and is isolated from the second vacuum chamber by closure of the door 510 separating the two vacuum chambers. Sample preparation may take place without damaging the vacuum provided within the second vacuum chamber. The sample holder is in contact with, and is cooled by, the first cooling unit (34a, 40a) at this time. The first vacuum chamber may then be pumped to a vacuum level commensurate with the vacuum within the second vacuum chamber in anticipation of sample delivery to the latter. Figure 15B: Next, the door 510 separating the first vacuum chamber from the second vacuum chamber is opened to allow sample delivery to the latter. Figure 15C: Next, the delivery mechanism transports the prepared sample from the first vacuum chamber to delivery position within the second vacuum chamber via the opened door 510. The manipulator arm 12 within the second vacuum chamber extends to move the clamp mechanism 500 towards the delivery position. Figure 15D: Subsequently, the manipulator arm 12 within the second vacuum chamber extends to place the clamp mechanism 500 in engagement with the sample holder 8. Figure 15E: Next, the manipulator arm 12 within the second vacuum chamber retracts to place the sample holder 8 at a desired analysis position ready for XPS analysis. The sample holder is in contact with, and is cooled by, the second cooling unit (34b, 40b) at this time. The delivery mechanism begins to return the first vacuum chamber via the opened door 510. Figure 15F: Next, the delivery mechanism fully returns to the first vacuum chamber and closes door 510. The sample holder is in contact with, and is cooled by, the second cooling unit (34b, 40b) at this time. Figure 15G: Next, XPS analysis upon the sample 10 takes place within the second vacuum chamber. The sample holder is in contact with, and is cooled by, the second cooling unit (34b, 40b) at this time. Figure 15H: Subsequently, after XPS analysis upon the sample 10 has ended, the door 510 re-opens and the delivery mechanism begins to advance towards the delivery position within the second vacuum chamber as the manipulator arm 12 extends to move the sample holder 8 to the same position. Figure 151: Next, the sample manipulator delivers the sample holder 8 to the delivery mechanism within the second vacuum chamber. Figure 15J: Next, the sample manipulator retracts from the delivery mechanism, releasing the delivered sample holder 8. Figure 15K: Finally, the delivery mechanism, returns the delivered sample holder 8 to the first vacuum chamber and closes the door 510 behind it. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example +1-10%. References A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein. [1] J.E. Graebner et al.: “Report on a second round robin measurement of the thermal conductivity of CVD diamond'; Diamond and Related Materials 7 (1998) 1589-1604. [2] Chen Xiao et al.: “Polishing of polycrystalline diamond using synergies between chemical and mechanical inputs: A review of mechanisms and processes”: Carbon, 196 (2022) 29-48. [3] Hu Luo et al. - “Polishing and planarization of single crystal diamonds: state-of-the-art and perspectives.”; 2021 Int. J. Extrem. Manuf. 3 022003.

Claims

1. A spectrometer apparatus for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising:a vacuum chamber for receiving a sample for pre-analysis preparation and / or for analysis;a cooling unit comprising a cooling contact surface presented within the vacuum chamber, wherein the cooling unit is configured to cool the cooling contact surface to a temperature below an ambient temperature;a moveable sample holder for holding the sample and comprising a coolable contact surface for separably contacting the cooling contact surface;wherein at least one of the cooling contact surface and the coolable contact surface comprises a thermal interface material the surface roughness of which is less than the surface roughness of the other of the cooling contact surface and the coolable contact surface.

2. A spectrometer according to any preceding claim wherein the thermal interface material of the at least one of the cooling contact surface and the coolable contact surface comprises the thermal conductance that is higher than the thermal conductance of the material of other of the cooling contact surface and the coolable contact surface.

3. A spectrometer according to any preceding claim wherein the thermal interface material comprises any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube, forming at least a part of one of the cooling contact surface and the coolable contact surface.

4. A spectrometer according to any preceding claim in which the coolable contact surface comprises the thermal interface material, wherein the moveable sample holder comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

5. A spectrometer according to any preceding claim in which the cooling contact surface comprises the thermal interface material, wherein the cooling unit comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

6. A spectrometer apparatus for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising:a vacuum chamber for receiving a sample for pre-analysis preparation and / or for analysis;a cooling unit comprising a cooling contact surface presented within the vacuum chamber, wherein the cooling unit is configured to cool the cooling contact surface to a temperature below an ambient temperature;a moveable sample holder for holding the sample and comprising a coolable contact surface for separably contacting the cooling contact surface;wherein at least a part of each of the cooling contact surface and the coolable contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube.

1. A spectrometer according to claim 6 wherein the moveable sample holder comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

8. A spectrometer according to claim 7 wherein the thermal interface material comprises the thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion of the moveable sample holder.

9. A spectrometer according to any of claims 6 to 8 wherein the cooling unit comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

10. A spectrometer according to claim 9 wherein the thermal interface material comprises the thermal conductance that is higher than the thermal conductance of the material of the thermally conductive body portion of the cooling unit.

11. A spectrometer for analysing particles emitted from, or transmitted through, a sample for analysis, the apparatus comprising:a vacuum chamber for receiving the sample holder upon a moveable sample holder for preanalysis preparation and / or for analysis;a cooling unit comprising a cooling contact surface presented within the vacuum chamber for forming a separable contact with a coolable contact surface of the sample holder;wherein the cooling unit is configured to cool the coolable contact surface to a temperature below an ambient temperature and the cooling contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube, forming at least a part of one of the cooling contact surface.

12. A spectrometer according to claim 11 wherein the cooling unit comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

13. A sample holder for holding a sample within a vacuum chamber of a spectrometer for pre-analysis preparation and / or for analysis of particles emitted from, or transmitted through, the sample , the spectrometer comprising a cooling unit comprising a cooling contact surface presented within thevacuum chamber, the cooling unit being configured to cool the cooling contact surface to a temperature below an ambient temperature, wherein:the sample holder comprises a coolable contact surface for separably contacting the cooling contact surface wherein the coolable contact surface comprises a thermal interface material comprising any of: poly-crystalline diamond; single-crystal diamond; graphene; boron arsenide; cubic boron nitride; carbon nanotube.

14. A sample holder according to claim 13, wherein the moveable sample holder comprises a thermally conductive body portion to a surface of which the thermal interface material is joined by a joining material comprising a Silver brazing material.

15. A spectrometer according to any preceding claim wherein the spectrometer is an X-ray photoelectron spectrometer, XPS, for illuminating the sample with X-rays therewith to generate an emission of photoelectrons from the sample for analysis.

16. A spectrometer according to any of preceding claims 1 to 14 wherein the spectrometer is an Ultraviolet photoelectron spectrometer for illuminating the sample with ultraviolet photons therewith to generate an emission of photoelectrons from the sample for analysis.

17. A spectrometer according to any of preceding claims 1 to 14 wherein the spectrometer is an Auger electron spectrometer for irradiating the sample with electrons therewith to generate an emission of electrons from the sample for analysis.

18. A spectrometer according to any of preceding claims 1 to 14 wherein the spectrometer is a Secondary Ion mass spectrometer, SIMS, for irradiating the sample with ions therewith to generate an emission of secondary ions from the sample for analysis.

19. A spectrometer according to any of preceding claims 1 to 14 wherein the spectrometer is an electron energy loss spectrometer, EELS.

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