TECHNICAL FIELD

[0001] The field of the invention relates to X-ray detectors or sensors; particularly those with suitability for detecting or sensing soft X-rays; more particularly those with suitability for detecting or sensing soft X-rays within the water window region.

BACKGROUND

[0002] The following discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of the application.

[0003] X-rays have enabled some of the most important scientific discoveries of the 20 ^th century, including verifying the wave-nature of photons, quantifying the structure of materials and enabling imaging of tissues and structures inside living organisms. X-ray photons can be broadly classified into hard and soft X-ray types.

[0004] Hard X-rays (those having photon energies >10 keV) are employed in diagnostic radiology, non-destructive testing, security systems and research. Meanwhile, soft X- rays (generally, those having photon energies falling within the range of approximately 0.1 - 10 keV) are utilised in microscopy, spectroscopy, and lithography.

[0005] A key emerging application for soft X-rays is their utilisation in the direct probing and imaging of biological specimens, such as wet proteins in their native conformations, and fixed living cells in their host aqueous media.

[0006] Understanding the true structural nature of such specimens in their native biological environment, is critical in the study of cell biology and biochemistry. The so-called “water window region” within the soft X-ray range of 200-600 eV is generally employed for such measurements to minimize the inherent absorption by the required aqueous media, compared to the absorption of biological specimens themselves. While this spectral region can be readily accessed with synchrotron radiation, recent advancements in non-synchrotron soft X-ray laser sources provides significant opportunities for portable and low-cost detection soft X-ray systems. However, for this to be realized, there is a need for high-performance, soft X-ray detectors that are highly sensitive to low energy X-rays, provide excellent spatial resolution, and preferably are non-toxic, and/or non-water soluble, and cost effective.

[0007] Conventional X-ray detectors suitable for detection of soft X-rays in the water window region exhibit low spatial resolution, have limited sensitivity and require complex fabrication procedures.

[0008] Existing soft X-ray detectors are based on an indirect detection mechanism, utilizing scintillating materials, such as polystyrene and Gd ₂ O ₂ S, that convert ionizing radiation into visible photons. This approach enables detection across multiple energy ranges and frame rates; however, its anisotropic response, complex synthesis procedures and limited resolution, due to the comparatively large thicknesses required, present major challenges.

[0009] Some of the limitations of implementing indirect detection mechanisms can be addressed utilising a direct detection approach, which offers significant fabrication and resolution benefits due to simpler device design demands, and lower thickness needs of the detecting material. However, a pre-requisite for selecting a suitable material for direct detection is achieving a high X-ray absorption coefficient, , where Z is the atomic number of the absorbing atoms, E is the X-ray incident energy, p is the density and A is the atomic mass of an atom.

[0010] Based on the criteria that high atomic mass generally produces higher absorption in low energy (soft) X-rays, and that soft X-rays are more strongly absorbed in thin materials than hard X-rays, a number of novel material systems have been recently identified based on organic, hybrid and inorganic semiconductors, among which materials in the organic-inorganic perovskite class have emerged as some of the most successful. ^[1]

[001 1 ] Within the soft X-ray region of 100 - 2500 eV, notable examples include ~20 nm thick CsPbBr ₃ nanocrystal films (via drop casting) ^[2] and ~53 nm CrSiTe ₃ ferromagnetic flakes (via mechanical exfoliation), ^[3] both of which have achieved excellent sensitivities >450 uGGy ^{- 1} cm ² at photon energies > 1 keV. However, these materials are less suitable for detection in the water window region, as they possess low energy resolution and low sensitivity values of <50 uGGy ^{- 1} cm ² . [0012] Accordingly, there is a substantial need for the development of high- performance, soft X-ray detectors that are highly sensitive to low energy X-rays within the water window region; that provide excellent spatial resolution; that may be easily fabricated from earth-abundant precursor materials; and preferably are non-toxic, and/or non-water soluble, and cost effective.

[0013] It is against this background that the present invention has been developed.

SUMMARY OF THE INVENTION

[0014] Herein is provided the use of tin mono-chalcogenide nanosheets for the detection of X-rays; processes for the manufacture of said tin mono-chalcogenide nanosheets; detectors comprising said tin mono-chalcogenide nanosheets; as well as devices comprising tin mono-chalcogenide nanosheet based detectors.

[0015] In one embodiment, the disclosure herein provides for the use of tin monochalcogenide nanosheets for the detection of X-rays; wherein the tin monochalcogenide nanosheets comprise tin mono-sulfide (SnS), or tin mono-selenide (SnSe).

[0016] In some embodiments, the tin mono-chalcogenide nanosheets are suitable for the detection of soft X-rays, preferably wherein the soft X-rays have photon energies falling in the range of 100 eV up to 5 keV, most preferably wherein the soft X-rays are in the water window region of 200 eV to 600 eV.

[0017] In one embodiment, the disclosure herein provides a detector for the detection of X-rays comprising, one or more tin mono-chalcogenide nanosheets and at least two electrodes; wherein the tin mono-chalcogenide nanosheets comprise tin mono-sulfide (SnS), or tin mono-selenide (SnSe).

[0018] In some embodiments, the thickness of the tin mono-chalcogenide nanosheets is within the range of 0.6 nm to 100 nm, preferably the thickness of the tin mono- chalcogenide nanosheets is within the range of 1 nm to 50 nm, most preferably the thickness of the tin mono-chalcogenide nanosheets is within the range of 1.5 to 10 nm, or 1 .7 nm to 9 nm.

[0019] In some embodiments of the detector of the present invention, the electrodes comprise a conductive material selected from the group consisting of; metals, metal alloys, noble metals, carbon, graphite, graphene, carbon nanofibers, carbon nanotubes, fullerenes, molecular wires, conducting polymers, polythiophenes, and Poly(3,4- ethylenedioxythiophene) (PEDOT), including hybrid or composite conductive materials or mixtures of any one or more members of the aforesaid group; preferably wherein the electrodes comprise a noble metal, most preferably wherein the electrodes comprise gold.

[0020] In some embodiments, the detector further comprises a support substrate upon which the tin mono-chalcogenide nanosheets are supported, wherein the support substrate comprises an electrically non-conductive support material suitably transparent to X-rays, selected from the group consisting of; flexible materials, rigid materials, polymeric materials, Polydimethylsiloxane (PDMS), polyimides, polycarbonates, Kapton, ceramic materials, glasses, silicate glasses and non-silicate glasses, including hybrid or composite support materials or mixtures of any one or more members of the aforesaid group; preferably wherein the support substrate is, or comprises, Si/SiO2.

[0021] In some embodiments of the detector of the present invention, the electrodes are fabricated via an electron-beam lithography and metal evaporation system.

[0022] In some embodiments, the thickness of the electrodes is within the range of 1 nm to 500 nm, preferably the thickness of the electrodes is within the range of 50 nm to 250 nm, most preferably the thickness of the electrodes is approximately 100 nm.

[0023] In some embodiments, the peak response of the detector is within the range of 100 eV to 1 keV, preferably within the range of 150 eV to 900 eV, most preferably within the range of 200 eV to 600 eV; or the peak response of the detector is approximately 700 eV or approximately 600 eV.

[0024] In some embodiments, the peak sensitivity of the detector to X-rays at a bias voltage of 1 V is within the range of 0.5x10 ⁴ to preferably within the range of 1x10 ⁴ to 3x10 ⁴ most preferably within the range of 1.1x10 ⁴ to or the peak sensitivity of the detector to X-rays at a bias voltage of

1 V is approximately 1 .15x10 ⁴ or approximately 2.55x10 ⁴

[0025] In some embodiments, the on/off transient response times of the detector to soft X-rays with photon energies in the range of 100 eV to 1 keV have; (i) a rise time within the range of approximately 2ms to approximately 7ms; and/or

(ii) a fall time within the range of approximately 1 ms to approximately 4ms; and/or

(iii) an average rise time of approximately 3ms to approximately 4ms, preferably an average rise time of approximately 3.4ms; and/or

(iv)an average fall time of approximately 2ms to approximately 3ms, preferably an average fall time of approximately 2.5ms.

[0026] In some embodiments, the on/off transient response times of the detector to soft X-rays with a photon energy of 600 eV have a rise time of approximately 7ms and a fall time of approximately 2ms.

[0027] In preferred embodiments of the detector, the one or more tin monochalcogenide nanosheets are tin mono-sulfide (SnS) nanosheets.

[0028] In one embodiment, the disclosure herein provides a device comprising at least one detector of the present invention.

[0029] In some embodiments, the device is a sensor, or a detector module, or a detector package, or an array of detectors, or a microscope, or an imaging device, or a dose rate measurement device, or a real-time dose rate measurement device.

[0030] In one embodiment, the disclosure herein provides a process when used for fabricating the detector of the invention, wherein the process comprises;

(i) synthesizing a tin mono-chalcogenide nanosheet by exposing molten liquid tin to a hydrogen chalcogenide gas, and thereby producing a tin mono-chalcogenide nanosheet at the surface of the molten liquid tin; wherein the hydrogen chalcogenide gas is selected from the group comprising hydrogen sulfide (H2S) gas and hydrogen selenide (H2Se) gas;

(ii) exfoliating the tin mono-chalcogenide nanosheet from the surface of the molten liquid tin;

(iii) transferring the exfoliated tin mono-chalcogenide nanosheet to a suitable support substrate, wherein the support substrate is electrically non-conductive and suitably transparent to X-rays, preferably wherein the support substrate is, or comprises, Si/SiO2; and

(iv) providing at least two electrodes, in electrical contact with the supported tin mono-chalcogenide nanosheet.

[0031] In some embodiments, providing at least two electrodes in accordance with step (iv) of the process of the invention, comprises printing metal electrodes on the surface of the supported tin mono-chalcogenide nanosheet via an electron-beam lithography and metal evaporation system, preferably wherein the metal comprises a noble metal, most preferably wherein the metal comprises gold.

[0032] In a preferred embodiment, the hydrogen chalcogenide gas in step (i) of the process of the invention, is hydrogen sulfide (H2S) gas, and the tin mono-chalcogenide nanosheet produced in the process is a tin mono-sulfide (SnS) nanosheet.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further features of the present invention are more fully described in the following description of several non-limiting embodiments thereof. This description is included solely for the purposes of exemplifying the present invention. It should not be understood as a restriction on the broad summary, disclosure or description of the invention as set out above. The description will be made with reference to the accompanying drawings in which:

Figure 1 depicts the characterization of tin mono-sulfide (SnS) nanosheets of the invention, as: (a) Raman spectra of the SnS thin sheets measured under ambient conditions showing associated vibrational peak shifts; (b) The attenuation coefficient or absorption (left axis) and corresponding average photon absorption range (right axis) as a function of photon energy in the soft X-rays range; (c) Theoretically derived Half Value Layer (HVL) values at different soft X-ray photon energy levels; (d) Comparison of absorption coefficients of SnS, CsPbBr ₃ , conventional LiF, Allose (as an example of an Organic molecule), and water.

Figure 2 depicts the investigation of the attenuation properties of SnS as: (a) A plot of Average Range as a function of photon energy (keV). Using a second order approximation, the average photon range for photon energies less than 1 keV were predicted via extrapolation as NIST data is only available from 1 keV. At 1 keV, around 250 nm of SnS is required to obtain an excellent sensitivity. For photon energies lower than 1 keV the arrows (inset) show the extrapolation, and indicate that few nanometres are sufficient to effectively absorb most of the soft X-rays; (b) A plot of the absorption coefficient of different perovskites and conventional materials at different X-ray photon energies. SnS is better than halide perovskites at relatively low photon energies, indicating excellent absorption properties for soft X-rays. This can be linked to higher densities of SnS (i.e. 5.22g/cm ³ ); (c) A plot of attenuation efficiency as a function of thickness comparing SnS with other halide perovskites, and indicating that lower thickness of SnS is required compared to other halide perovskites, in order to absorb low energy soft X-rays.

Figure 3 depicts the performance of a soft X-ray detection device with a thickness of 1.7 nm in accordance with the present invention as: (a) An Atomic Force Microscopy (AFM) image of the SnS nanosheets along with the profile showing a thickness of 1.7 nm; (b) The device response, in terms of drain current, to several photon energies; (c) Experimentally measured Photon Flux values as a function of Photon Energy.

Figure 4 depicts the performance of a soft X-ray detection device with a thickness of approximately 9 nm in accordance with the present invention as: (a) Comparison of Ids response for the water window region and energies greater than 1 KeV, indicating an excellent response around 600 eV; (b) IV curves at photon energies between 100eV to 1 KeV. A maximum photocurrent is observed around 600 eV followed by a rapid decrease with increasing photon energies; (c) The drain current Ids as a function of dose rate at several bias voltages. Sensitivity values are extracted from the slope of these photocurrents; (d) The sensitivity values at several bias voltages showing a linear increase with increasing bias voltages. Dose Rates for photon energies above 600 eV are not included; (e) Time-dependent Ids responses at Vb = 1 V for several Australian Synchrotron photon fluxes; (f) Temporal response of the detection device at 600 eV and Vb = 1 V; indicating rise (90% of the saturated X-ray signal) and fall (10% of the saturated X-ray signal) times of 7ms and 2 ms respectively, representing a significant improvement over other reported halide perovskites and ferromagnetic soft X-ray detectors.

Figure 5 depicts the performance of a soft X-ray detection device with a thickness of approximately 9 nm in accordance with the present invention as: (a) AFM thickness plot; (b) l-V curves at photon energies between 100eV to 1 keV, showing a maximum signal at 700 eV for the 1.7 nm thickness device in Figure 2; (c) The l-T data as a function of photon energies, showing maximum photocurrent is obtained at 700 eV; (d) The drain current as a function of dose rate at fixed bias voltages, linearly fitted to obtain the sensitivity of the device, (e) Sensitivity values as a function of drain voltage, showing the sensitivity values can reach up to “ around

1 V and that high sensitivity in SnS detector is related to low noise current and excellent charge collection efficiencies.

Figure 6 is a plot comparing sensitivity values for existing perovskite materials, newly discovered CrSiTe ₃ and detectors of the present invention under exposure to hard and soft X-rays.

Figure 7 is a plot of the response times of the SnS based soft X-ray detector embodiment of the present invention as a function of different incident photon energies. An average rise time of ~ 3.43 ms and fall time of ~ 2.53 ms for soft X-ray wavelengths between 100 eV to 1 keV was observed. The X-ray detector responds faster and almost the same on average, at wavelengths other than 600 to 700 eV. This response time is about five times better than CrSiTe ₃ and CsPbBr ₃ soft X-ray detectors due to an effective electron replenishment process.

Figure 8 is a schematic illustration of the key photolithography procedures used to fabricate detectors in accordance with the present invention.

DEFINITIONS

[0034] Throughout this specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. [0035] As used herein, the term “soft X-rays” and grammatical variations of the term, refers generally, to X-rays having photon energies falling within the range of approximately 0.1 - 10 keV.

[0036] As used herein, the terms “water window” and “water window region” and grammatical variations of the term(s), refer to the region within the soft X-ray region, having photon energies that are generally not absorbed by water, and/or to X-rays having photon energies falling within the range of approximately 200-600 eV.

[0037] As used herein, the term “suitably transparent to X-rays” and grammatical variations of the term, as it pertains to electrically non-conductive support materials utilised in the detectors of the present invention, generally refers to the property of the support material being sufficiently transparent to (or to the property of the support material being sufficiently non-absorbing of) X-rays having photon energies within the region for which the detector is being applied to the detection of X-rays, that the detector may operate effectively for the particular application in which it is utilised.

[0038] As used herein, the term “flexible” and “rigid” are to be given their plain English language meaning, as defined in a common English language dictionary.

[0039] Other definitions for selected terms used herein may be found within the detailed description of the invention and apply throughout. Unless otherwise defined, all other scientific and technical terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which the invention belongs.

DETAILED DESCRIPTION

[0040] The disclosure herein provides for a new class of direct soft X-ray detectors, based on tin mono-chalcogenide nanosheets, and advantageously possessing the properties of tunable sensitivity, high photon absorption efficiency in the soft X-ray region, and/or high sensitivity in the water window region. The tin mono-chalcogenide nanosheet detectors of the invention also possess the advantages of being ultrathin and easily fabricated from earth-abundant starting materials, and/or having low toxicity, and/or having low water solubility.

[0041] Although the preferred embodiment described herein employs tin mono-sulfide nanosheets, the skilled addressee will understand that the process for fabrication of the tin mono-chalcogenide nanosheets of the present invention may be utilized without undue burden, to produce alternative tin mono-chalcogenide nanosheets, including tin mono-selenide nanosheets, to produce that similar advantageous performance properties in the soft X-ray sensors of the invention, including tunable sensitivity, high photon absorption efficiency in the soft X-ray region, and/or high sensitivity in the water window region.

[0042] The high photon absorption efficiency of the tin mono-chalcogenide nanosheets of the present invention in the soft X-ray region enables the fabrication of soft X-ray detectors which exhibit excellent sensitivity values in the order of 10 ⁴ at peak energies of approximately 600 eV. The peak signal is found to be sensitive to the number of stacked tin mono-chalcogenide nanosheet layers, with thicker tin monochalcogenide nanosheet assemblies yielding a peak response at higher energies and with peak sensitives of over 2.5 x 10 ⁴

[0043] These results demonstrate the excellent performance of tin mono-chalcogenide nanosheet based soft X-ray detectors compared to existing direct soft X-ray detectors, including that of the emerging organic-inorganic perovskite class of materials.

[0044] The tin mono-chalcogenide nanosheets of the present invention possess high photon absorption coefficients, enabling the fabrication of ultrathin soft X-ray detectors with excellent sensitivities of approximately 10 ⁴ Notably, the sensitivities of the tin mono-chalcogenide soft X-ray detectors of the present invention are an order of magnitude higher than the best current metal halide perovskite analogues, while also possessing an enhanced response time of approximately 7ms compared to tens of ms for conventional soft X-ray detectors. ^[2] Increasing the thickness of the tin mono-chalcogenide nanosheets of the present invention efficiently enables detection of photons at energies of more than 1 keV, and provides X-ray detectors that exhibit tuneable sensitivity factors across the soft X-ray region.

[0045] Conventional fabrication methods for nanosheets utilise chemical/physical vapour deposition or 2D exfoliation synthesis techniques. However, such methods are unsuitable for the fabrication of ultrathin tin mono-chalcogenide nanosheets because the interlayer interaction caused by the lone pair of electrons from the chalcogen are stronger than the van der Waals (vdW) forces between tin mono-chalcogenide layers.

[0046] To overcome this issue, a liquid metal based exfoliation method has been developed to obtain high-quality, large area tin mono-chalcogenide nanosheets with controlled thicknesses. ^[4] The method relies on the chalcogenization of molten tin followed by the isolation of tin mono-chalcogenide nanosheets through a vdW exfoliation technique. This is achieved by bringing a suitable support substrate into contact with a chalcogenized liquid metal surface, where the polar surface drives delamination of the tin mono-chalcogenide nanosheets from the surface of the non- polar molten tin. In a preferred embodiment, Si/SiO ₂ (300 nm) support substrates are utilized for this purpose, with the polar SiO ₂ surface enabling highly efficient exfoliation of tin mono-chalcogenide nanosheets.

[0047] Although the preferred embodiment described herein employs Si/SiO2 support substrates, the skilled addressee will understand that alternative support substrates may be utilized, including support substrates comprising any electrically non-conductive support material suitably transparent to X-rays, for example, those selected from the group consisting of; flexible materials, rigid materials, polymeric materials, PDMS, polyimides, polycarbonates, Kapton, ceramic materials, glasses, silicate glasses and non-silicate glasses, as well as hybrid or composite support materials or mixtures of any one or more members of the aforesaid group, to produce similar advantageous performance properties in the soft X-ray sensors of the invention, including tunable sensitivity, high photon absorption efficiency in the soft X-ray region, and/or high sensitivity in the water window region.

[0048] As an initial investigation into the potential of tin mono-chalcogenide nanosheets for X-ray detection, the NIST photon cross section database was used to reveal the X- ray absorption properties of tin mono-sulfide (SnS). The X-ray absorption coefficients of SnS across the soft X-ray region have been extracted from this database and are shown in Figure 1 b. At 1 keV, SnS possesses an absorption coefficient of 0.0036/nm, which correlates to an X-ray absorption length (1 Ze) of -278 nm. Given that 1 keV is the minimum reported photon energy for X-ray on the NIST database, a second order approximation was used to extrapolate the absorption range across photon energies less than 1 keV (Figure 2a). The increased absorption coefficients predicted at these energies indicate that the nanometer-thick SnS sheets should effectively absorb soft X- rays within the water window region.

[0049] This is also reflected in the half-value layer “HVL” plot of SnS (Figure 1 c), which is generally used to evaluate the strength of a material’s ability to absorb half of the radiation. The high attenuation efficiency of SnS compared to other suitable materials, including perovskites, is further shown in Figure 2. In order to illustrate the significant application potential of tin mono-chalcogenide nanosheets in the water window, a comparison plot containing the absorption coefficients of SnS, CsPbBr ₃ perovskite, LiF, allose (as an example of an organic molecule), and water, are shown in Figure 1d. The extrapolation of this data into the water window showcases the higher absorption coefficient of SnS as compared to these other materials. A broader comparison to several well-known (Pb and Bi) metal halide perovskites and the archetypal amorphous Se (Figure 2b), further shows that SnS is a better candidate for soft X-ray detection than all of these materials. Despite Sn being a lighter element than Pb and Bi, the binary structure of SnS supports a higher density of 5.22 g/cm ³ than metal halide perovskites (which generally exhibit densities of 4 to 5 g/cm ³ ), making tin-monochalcogenides extremely effective attenuators of X-rays (Figure 2c).

[0050] Based on the promising outcomes of this theoretical investigation, X-ray detectors were fabricated by depositing gold electrodes within a two-terminal configuration directly on exfoliated 1 .7 nm thick SnS nanosheets on Si/SiC>2 substrates (Figure 3a). X-ray detection was subsequently achieved by exposing the active areas of the devices to soft X-rays with energies of between 100 eV to 1 keV and monitoring the change in the detector device photocurrent Al (= liight - Idark). In this photon energy range, the soft X-ray beam at the Australian Synchrotron is adequately stable and possesses high intrinsic energy resolution, providing a suitable source of soft X-rays to enable accurate assessment of the X-ray detector of the present invention.

[0051] Although the electrodes provided in the exemplary detectors of the present specification are gold electrodes, the skilled addressee will understand that a wide range of alternative materials may utilized to provide electrodes, without departing from the present invention. For example, electrodes may be provided using any suitably conductive material, including, without limitation, metals, metal alloys, noble metals, carbon, graphite, graphene, carbon nanofibers, carbon nanotubes, fullerenes, molecular wires, conducting polymers, polythiophenes, and PEDOT, as well as hybrid or composite conductive materials or mixtures of any one or more of the aforementioned.

[0052] Similarly, although the electrodes provided in the exemplary detectors of the present specification were fabricated via an electron-beam lithography and metal evaporation system, the skilled addressee will understand that numerous alternative means for providing electrodes exist within the art, and may be utilized to provide electrodes without departing from the present invention. For example, electrodes may be provided using, without limitation, photolithography, X-ray lithography, extreme ultraviolet lithography, light coupling nanolithography, scanning probe microsope lithography, nanoimprint lithography or dip-pen nanolithography.

[0053] An ideal X-ray detector requires a large dynamic range and an energy-dependent differentiable response, in order to detect variations in dose rate and discriminate the signal based on photon energy, respectively. As shown in Figure 3b, the SnS based soft X-ray detector in accordance with an embodiment of the present invention, operated at 1 V, shows a differential response to different incident photon energies with a peak response at 600 eV. The measurable Ids signal (~0.76 nA) at 100 eV (Figure 3b) is three times greater than the noise level, which is well within the International Union of Pure and Applied Chemistry standards (IUPAC). I _ds is further increased at higher X-ray energies, reaching a peak value of 8.3 nA at 600 eV. This peak signal is almost 80 times larger compared to that measured at 1.1 keV (Figure 4a), indicating the device’s excellent response in the water window region. The large energy-dependent differentiable signals in the water window region observed with the detector of the present invention represent a major improvement compared to other direct soft X-ray detectors ^[2] , for which two-fold lower currents have been observed.

[0054] The current-voltage (IV) curves of the detectors of the present invention were also measured and are presented in Figure 4b for different X-ray energies. These show the formation of a Schottky junction at the SnS and gold electrodes interface, which indicates that the device performance can be further improved by designing ohmic contacts through work function and chemical potential matching between the tin monochalcogenide and the electrodes. The IV curves also clearly exhibit a peak photocurrent at a 600 eV X-ray energy.

[0055] Without wishing to be bound by theory, it is believed that the photocurrent is dependent directly on the dose and attenuation coefficient of the tin monochalcogenide at various X-ray energies. The measured X-ray photon flux across the energy region of interest is depicted in Figure 3c, showing a peak flux at X-ray energies of between 600 and 700 eV. When this flux profile is considered in light of the decreasing attenuation coefficient of the SnS at higher X-ray photon energies, the peak performance of the detector at 600 eV can be understood. [0056] The key measure of performance for X-ray detectors is sensitivity (S) and is defined as Ip is the photocurrent, D is the dose rate and A is the active device area. To calculate the sensitivity of the detectors of the present invention, the drain current Ids as a function of dose rate at fixed bias voltages in the water window region was determined. The Ids shows an almost linear response to the dose rate. Achieving good linearity is a desirable property for detector fabrication because linear responses can minimise errors in the output signal. Using the extracted slope values from Figure 4c, the sensitivity at several bias voltages are shown in Figure 4d. At a bias voltage of 1 V, the sensitivity values can reach up to

[0057] Increasing the thickness of the tin monochalcogenide nanosheet assemblies enables improved absorption of soft X-rays at a given energy, further enhancing the sensitivity values. SnS nanosheets of approximately 9 nm in thickness demonstrate this property (Figure 5a). While the obtained l-V trends (Figure 5b) of these devices are similar to those prepared using the 1.7 nm thick SnS nanosheets, the increase in thickness shifts the peak signal to 700 eV (Figure 5c). This is due to the increased absorption by the thicker SnS nanosheets, indicating efficient detection of > 1 keV could easily be achieved by further increasing the thickness of the SnS nanosheets, and demonstrating that peak sensitivity of the tin monochalcogenide nanosheet based detectors of the present invention may be tuned by varying the thickness of the nanosheets.

[0058] For example, in accordance with embodiments of the present invention, the thickness of the tin mono-chalcogenide nanosheets may be 0.6 nm, 0.7 nm, 0.8 nm, 0.9 nm, 1 nm, 1.1 nm, 1.2 nm, 1.3 nm, 1.4 nm, 1.5 nm, 1.6 nm, 1.7 nm, 1.8 nm, 1.9 nm, 2 nm, 2.1 nm, 2.2 nm, 2.3 nm, 2.4 nm, 2.5 nm, 2.6 nm, 2.7 nm, 2.8 nm, 2.9 nm, 3 nm, 3.1 nm, 3.2 nm, 3.3 nm, 3.4 nm, 3.5 nm, 3.6 nm, 3.7 nm, 3.8 nm, 3.9 nm, 4 nm, 4.1 nm, 4.2 nm, 4.3 nm, 4.4 nm, 4.5 nm, 4.6 nm, 4.7 nm, 4.8 nm, 4.9 nm, 5 nm, 5.1 nm, 5.2 nm, 5.3 nm, 5.4 nm, 5.5 nm, 5.6 nm, 5.7 nm, 5.8 nm, 5.9 nm, 6 nm, 6.1 nm, 6.2 nm, 6.3 nm, 6.4 nm, 6.5 nm, 6.6 nm, 6.7 nm, 6.8 nm, 6.9 nm, 7 nm, 7.1 nm, 7.2 nm, 7.3 nm, 7.4 nm, 7.5 nm, 7.6 nm, 7.7 nm, 7.8 nm, 7.9 nm, 8 nm, 8.1 nm, 8.2 nm, 8.3 nm, 8.4 nm, 8.5 nm, 8.6 nm, 8.7 nm, 8.8 nm, 8.9 nm, 9 nm, 9.1 nm, 9.2 nm, 9.3 nm, 9.4 nm, 9.5 nm, 9.6 nm, 9.7 nm, 9.8 nm, 9.9 nm, 10 nm, 10.1 nm, 10.2 nm, 10.3 nm, 10.4 nm, 10.5 nm, 10.6 nm, 10.7 nm, 10.8 nm, 10.9 nm, 11 nm, 11.1 nm, 11.2 nm, 11.3 nm, 11.4 nm, 11.5 nm, 11.6 nm, 11 .7 nm, 11 .8 nm, 11 .9 nm, 12 nm, 12.1 nm, 12.2 nm, 12.3 nm, 12.4 nm, 12.5 nm, 12.6 nm, 12.7 nm, 12.8 nm, 12.9 nm, 13 nm, 13.1 nm, 13.2 nm, 13.3 nm, 13.4 nm, 13.5 nm, 13.6 nm, 13.7 nm, 13.8 nm, 13.9 nm, 14 nm, 14.1 nm, 14.2 nm, 14.3 nm, 14.4 nm,

14.5 nm, 14.6 nm, 14.7 nm, 14.8 nm, 14.9 nm, 15 nm, 15.1 nm, 15.2 nm, 15.3 nm, 15.4 nm, 15.5 nm, 15.6 nm, 15.7 nm, 15.8 nm, 15.9 nm, 16 nm, 16.1 nm, 16.2 nm, 16.3 nm,

16.4 nm, 16.5 nm, 16.6 nm, 16.7 nm, 16.8 nm, 16.9 nm, 17 nm, 17.1 nm, 17.2 nm, 17.3 nm, 17.4 nm, 17.5 nm, 17.6 nm, 17.7 nm, 17.8 nm, 17.9 nm, 18 nm, 18.1 nm, 18.2 nm,

18.3 nm, 18.4 nm, 18.5 nm, 18.6 nm, 18.7 nm, 18.8 nm, 18.9 nm, 19 nm, 19.1 nm, 19.2 nm, 19.3 nm, 19.4 nm, 19.5 nm, 19.6 nm, 19.7 nm, 19.8 nm, 19.9 nm, 20 nm, 20.1 nm,

20.2 nm, 20.3 nm, 20.4 nm, 20.5 nm, 20.6 nm, 20.7 nm, 20.8 nm, 20.9 nm, 21 nm, 21.1 nm, 21.2 nm, 21.3 nm, 21.4 nm, 21.5 nm, 21.6 nm, 21.7 nm, 21.8 nm, 21.9 nm, 22 nm,

22.1 nm, 22.2 nm, 22.3 nm, 22.4 nm, 22.5 nm, 22.6 nm, 22.7 nm, 22.8 nm, 22.9 nm, 23 nm, 23.1 nm, 23.2 nm, 23.3 nm, 23.4 nm, 23.5 nm, 23.6 nm, 23.7 nm, 23.8 nm, 23.9 nm, 24 nm, 24.1 nm, 24.2 nm, 24.3 nm, 24.4 nm, 24.5 nm, 24.6 nm, 24.7 nm, 24.8 nm, 24.9 nm, 25 nm, 25.1 nm, 25.2 nm, 25.3 nm, 25.4 nm, 25.5 nm, 25.6 nm, 25.7 nm, 25.8 nm, 25.9 nm, 26 nm, 26.1 nm, 26.2 nm, 26.3 nm, 26.4 nm, 26.5 nm, 26.6 nm, 26.7 nm,

26.8 nm, 26.9 nm, 27 nm, 27.1 nm, 27.2 nm, 27.3 nm, 27.4 nm, 27.5 nm, 27.6 nm, 27.7 nm, 27.8 nm, 27.9 nm, 28 nm, 28.1 nm, 28.2 nm, 28.3 nm, 28.4 nm, 28.5 nm, 28.6 nm,

28.7 nm, 28.8 nm, 28.9 nm, 29 nm, 29.1 nm, 29.2 nm, 29.3 nm, 29.4 nm, 29.5 nm, 29.6 nm, 29.7 nm, 29.8 nm, 29.9 nm, 30 nm, 30.1 nm, 30.2 nm, 30.3 nm, 30.4 nm, 30.5 nm,

30.6 nm, 30.7 nm, 30.8 nm, 30.9 nm, 31 nm, 31.1 nm, 31 .2 nm, 31 .3 nm, 31 .4 nm, 31 .5 nm, 31.6 nm, 31.7 nm, 31.8 nm, 31.9 nm, 32 nm, 32.1 nm, 32.2 nm, 32.3 nm, 32.4 nm,

32.5 nm, 32.6 nm, 32.7 nm, 32.8 nm, 32.9 nm, 33 nm, 33.1 nm, 33.2 nm, 33.3 nm, 33.4 nm, 33.5 nm, 33.6 nm, 33.7 nm, 33.8 nm, 33.9 nm, 34 nm, 34.1 nm, 34.2 nm, 34.3 nm,

34.4 nm, 34.5 nm, 34.6 nm, 34.7 nm, 34.8 nm, 34.9 nm, 35 nm, 35.1 nm, 35.2 nm, 35.3 nm, 35.4 nm, 35.5 nm, 35.6 nm, 35.7 nm, 35.8 nm, 35.9 nm, 36 nm, 36.1 nm, 36.2 nm,

36.3 nm, 36.4 nm, 36.5 nm, 36.6 nm, 36.7 nm, 36.8 nm, 36.9 nm, 37 nm, 37.1 nm, 37.2 nm, 37.3 nm, 37.4 nm, 37.5 nm, 37.6 nm, 37.7 nm, 37.8 nm, 37.9 nm, 38 nm, 38.1 nm,

38.2 nm, 38.3 nm, 38.4 nm, 38.5 nm, 38.6 nm, 38.7 nm, 38.8 nm, 38.9 nm, 39 nm, 39.1 nm, 39.2 nm, 39.3 nm, 39.4 nm, 39.5 nm, 39.6 nm, 39.7 nm, 39.8 nm, 39.9 nm, 40 nm, 40.1 nm, 40.2 nm, 40.3 nm, 40.4 nm, 40.5 nm, 40.6 nm, 40.7 nm, 40.8 nm, 40.9 nm, 41 nm, 41.1 nm, 41.2 nm, 41.3 nm, 41.4 nm, 41.5 nm, 41.6 nm, 41.7 nm, 41.8 nm, 41.9 nm, 42 nm, 42.1 nm, 42.2 nm, 42.3 nm, 42.4 nm, 42.5 nm, 42.6 nm, 42.7 nm, 42.8 nm,

42.9 nm, 43 nm, 43.1 nm, 43.2 nm, 43.3 nm, 43.4 nm, 43.5 nm, 43.6 nm, 43.7 nm, 43.8 nm, 43.9 nm, 44 nm, 44.1 nm, 44.2 nm, 44.3 nm, 44.4 nm, 44.5 nm, 44.6 nm, 44.7 nm, 44.8 nm, 44.9 nm, 45 nm, 45.1 nm, 45.2 nm, 45.3 nm, 45.4 nm, 45.5 nm, 45.6 nm, 45.7 nm, 45.8 nm, 45.9 nm, 46 nm, 46.1 nm, 46.2 nm, 46.3 nm, 46.4 nm, 46.5 nm, 46.6 nm,

46.7 nm, 46.8 nm, 46.9 nm, 47 nm, 47.1 nm, 47.2 nm, 47.3 nm, 47.4 nm, 47.5 nm, 47.6 nm, 47.7 nm, 47.8 nm, 47.9 nm, 48 nm, 48.1 nm, 48.2 nm, 48.3 nm, 48.4 nm, 48.5 nm,

48.6 nm, 48.7 nm, 48.8 nm, 48.9 nm, 49 nm, 49.1 nm, 49.2 nm, 49.3 nm, 49.4 nm, 49.5 nm, 49.6 nm, 49.7 nm, 49.8 nm, 49.9 nm, 50 nm, 50.1 nm, 50.2 nm, 50.3 nm, 50.4 nm,

50.5 nm, 50.6 nm, 50.7 nm, 50.8 nm, 50.9 nm, 51 nm, 51.1 nm, 51.2 nm, 51.3 nm, 51.4 nm, 51.5 nm, 51.6 nm, 51.7 nm, 51.8 nm, 51.9 nm, 52 nm, 52.1 nm, 52.2 nm, 52.3 nm,

52.4 nm, 52.5 nm, 52.6 nm, 52.7 nm, 52.8 nm, 52.9 nm, 53 nm, 53.1 nm, 53.2 nm, 53.3 nm, 53.4 nm, 53.5 nm, 53.6 nm, 53.7 nm, 53.8 nm, 53.9 nm, 54 nm, 54.1 nm, 54.2 nm,

54.3 nm, 54.4 nm, 54.5 nm, 54.6 nm, 54.7 nm, 54.8 nm, 54.9 nm, 55 nm, 55.1 nm, 55.2 nm, 55.3 nm, 55.4 nm, 55.5 nm, 55.6 nm, 55.7 nm, 55.8 nm, 55.9 nm, 56 nm, 56.1 nm,

56.2 nm, 56.3 nm, 56.4 nm, 56.5 nm, 56.6 nm, 56.7 nm, 56.8 nm, 56.9 nm, 57 nm, 57.1 nm, 57.2 nm, 57.3 nm, 57.4 nm, 57.5 nm, 57.6 nm, 57.7 nm, 57.8 nm, 57.9 nm, 58 nm,

58.1 nm, 58.2 nm, 58.3 nm, 58.4 nm, 58.5 nm, 58.6 nm, 58.7 nm, 58.8 nm, 58.9 nm, 59 nm, 59.1 nm, 59.2 nm, 59.3 nm, 59.4 nm, 59.5 nm, 59.6 nm, 59.7 nm, 59.8 nm, 59.9 nm, 60 nm, 60.1 nm, 60.2 nm, 60.3 nm, 60.4 nm, 60.5 nm, 60.6 nm, 60.7 nm, 60.8 nm,

60.9 nm, 61 nm, 61 .1 nm, 61 .2 nm, 61 .3 nm, 61 .4 nm, 61 .5 nm, 61 .6 nm, 61 .7 nm, 61 .8 nm, 61.9 nm, 62 nm, 62.1 nm, 62.2 nm, 62.3 nm, 62.4 nm, 62.5 nm, 62.6 nm, 62.7 nm,

62.8 nm, 62.9 nm, 63 nm, 63.1 nm, 63.2 nm, 63.3 nm, 63.4 nm, 63.5 nm, 63.6 nm, 63.7 nm, 63.8 nm, 63.9 nm, 64 nm, 64.1 nm, 64.2 nm, 64.3 nm, 64.4 nm, 64.5 nm, 64.6 nm,

64.7 nm, 64.8 nm, 64.9 nm, 65 nm, 65.1 nm, 65.2 nm, 65.3 nm, 65.4 nm, 65.5 nm, 65.6 nm, 65.7 nm, 65.8 nm, 65.9 nm, 66 nm, 66.1 nm, 66.2 nm, 66.3 nm, 66.4 nm, 66.5 nm,

66.6 nm, 66.7 nm, 66.8 nm, 66.9 nm, 67 nm, 67.1 nm, 67.2 nm, 67.3 nm, 67.4 nm, 67.5 nm, 67.6 nm, 67.7 nm, 67.8 nm, 67.9 nm, 68 nm, 68.1 nm, 68.2 nm, 68.3 nm, 68.4 nm,

68.5 nm, 68.6 nm, 68.7 nm, 68.8 nm, 68.9 nm, 69 nm, 69.1 nm, 69.2 nm, 69.3 nm, 69.4 nm, 69.5 nm, 69.6 nm, 69.7 nm, 69.8 nm, 69.9 nm, 70 nm, 70.1 nm, 70.2 nm, 70.3 nm,

70.4 nm, 70.5 nm, 70.6 nm, 70.7 nm, 70.8 nm, 70.9 nm, 71 nm, 71.1 nm, 71.2 nm, 71.3 nm, 71.4 nm, 71.5 nm, 71.6 nm, 71.7 nm, 71.8 nm, 71.9 nm, 72 nm, 72.1 nm, 72.2 nm,

72.3 nm, 72.4 nm, 72.5 nm, 72.6 nm, 72.7 nm, 72.8 nm, 72.9 nm, 73 nm, 73.1 nm, 73.2 nm, 73.3 nm, 73.4 nm, 73.5 nm, 73.6 nm, 73.7 nm, 73.8 nm, 73.9 nm, 74 nm, 74.1 nm,

74.2 nm, 74.3 nm, 74.4 nm, 74.5 nm, 74.6 nm, 74.7 nm, 74.8 nm, 74.9 nm, 75 nm, 75.1 nm, 75.2 nm, 75.3 nm, 75.4 nm, 75.5 nm, 75.6 nm, 75.7 nm, 75.8 nm, 75.9 nm, 76 nm, 76.1 nm, 76.2 nm, 76.3 nm, 76.4 nm, 76.5 nm, 76.6 nm, 76.7 nm, 76.8 nm, 76.9 nm, 77 nm, 77.1 nm, 77.2 nm, 77.3 nm, 77.4 nm, 77.5 nm, 77.6 nm, 77.7 nm, 77.8 nm, 77.9 nm, 78 nm, 78.1 nm, 78.2 nm, 78.3 nm, 78.4 nm, 78.5 nm, 78.6 nm, 78.7 nm, 78.8 nm, 78.9 nm, 79 nm, 79.1 nm, 79.2 nm, 79.3 nm, 79.4 nm, 79.5 nm, 79.6 nm, 79.7 nm, 79.8 nm, 79.9 nm, 80 nm, 80.1 nm, 80.2 nm, 80.3 nm, 80.4 nm, 80.5 nm, 80.6 nm, 80.7 nm,

80.8 nm, 80.9 nm, 81 nm, 81.1 nm, 81.2 nm, 81.3 nm, 81.4 nm, 81.5 nm, 81.6 nm, 81.7 nm, 81.8 nm, 81.9 nm, 82 nm, 82.1 nm, 82.2 nm, 82.3 nm, 82.4 nm, 82.5 nm, 82.6 nm,

82.7 nm, 82.8 nm, 82.9 nm, 83 nm, 83.1 nm, 83.2 nm, 83.3 nm, 83.4 nm, 83.5 nm, 83.6 nm, 83.7 nm, 83.8 nm, 83.9 nm, 84 nm, 84.1 nm, 84.2 nm, 84.3 nm, 84.4 nm, 84.5 nm, 84.6 nm, 84.7 nm, 84.8 nm, 84.9 nm, 85 nm, 85.1 nm, 85.2 nm, 85.3 nm, 85.4 nm, 85.5 nm, 85.6 nm, 85.7 nm, 85.8 nm, 85.9 nm, 86 nm, 86.1 nm, 86.2 nm, 86.3 nm, 86.4 nm, 86.5 nm, 86.6 nm, 86.7 nm, 86.8 nm, 86.9 nm, 87 nm, 87.1 nm, 87.2 nm, 87.3 nm, 87.4 nm, 87.5 nm, 87.6 nm, 87.7 nm, 87.8 nm, 87.9 nm, 88 nm, 88.1 nm, 88.2 nm, 88.3 nm, 88.4 nm, 88.5 nm, 88.6 nm, 88.7 nm, 88.8 nm, 88.9 nm, 89 nm, 89.1 nm, 89.2 nm, 89.3 nm, 89.4 nm, 89.5 nm, 89.6 nm, 89.7 nm, 89.8 nm, 89.9 nm, 90 nm, 90.1 nm, 90.2 nm, 90.3 nm, 90.4 nm, 90.5 nm, 90.6 nm, 90.7 nm, 90.8 nm, 90.9 nm, 91 nm, 91.1 nm, 91.2 nm, 91.3 nm, 91.4 nm, 91.5 nm, 91.6 nm, 91.7 nm, 91.8 nm, 91.9 nm, 92 nm, 92.1 nm, 92.2 nm, 92.3 nm, 92.4 nm, 92.5 nm, 92.6 nm, 92.7 nm, 92.8 nm, 92.9 nm, 93 nm, 93.1 nm, 93.2 nm, 93.3 nm, 93.4 nm, 93.5 nm, 93.6 nm, 93.7 nm, 93.8 nm, 93.9 nm, 94 nm, 94.1 nm, 94.2 nm, 94.3 nm, 94.4 nm, 94.5 nm, 94.6 nm, 94.7 nm, 94.8 nm, 94.9 nm, 95 nm, 95.1 nm, 95.2 nm, 95.3 nm, 95.4 nm, 95.5 nm, 95.6 nm, 95.7 nm, 95.8 nm, 95.9 nm, 96 nm, 96.1 nm, 96.2 nm, 96.3 nm, 96.4 nm, 96.5 nm, 96.6 nm, 96.7 nm, 96.8 nm,

96.9 nm, 97 nm, 97.1 nm, 97.2 nm, 97.3 nm, 97.4 nm, 97.5 nm, 97.6 nm, 97.7 nm, 97.8 nm, 97.9 nm, 98 nm, 98.1 nm, 98.2 nm, 98.3 nm, 98.4 nm, 98.5 nm, 98.6 nm, 98.7 nm,

98.8 nm, 98.9 nm, 99 nm, 99.1 nm, 99.2 nm, 99.3 nm, 99.4 nm, 99.5 nm, 99.6 nm, 99.7 nm, 99.8 nm, 99.9 nm, or 100 nm.

[0059] Due to the tuneability of the peak response of the detectors with varying tin mono-chalcogenide thicknesses in accordance with embodiments of the present invention, the peak response of the tin mono-chalcogenide nanosheet based detectors of the invention may be approximately 100 eV, 110 eV, 120 eV, 130 eV, 140 eV, 150 eV, 160 eV, 170 eV, 180 eV, 190 eV, 200 eV, 210 eV, 220 eV, 230 eV, 240 eV, 250 eV, 260 eV, 270 eV, 280 eV, 290 eV, 300 eV, 310 eV, 320 eV, 330 eV, 340 eV, 350 eV, 360 eV, 370 eV, 380 eV, 390 eV, 400 eV, 410 eV, 420 eV, 430 eV, 440 eV, 450 eV, 460 eV, 470 eV, 480 eV, 490 eV, 500 eV, 510 eV, 520 eV, 530 eV, 540 eV, 550 eV, 560 eV, 570 eV, 580 eV, 590 eV, 600 eV, 610 eV, 620 eV, 630 eV, 640 eV, 650 eV, 660 eV, 670 eV, 680 eV, 690 eV, 700 eV, 710 eV, 720 eV, 730 eV, 740 eV, 750 eV, 760 eV, 770 eV, 780 eV, 790 eV, 800 eV, 810 eV, 820 eV, 830 eV, 840 eV, 850 eV, 860 eV, 870 eV, 880 eV,

890 eV, 900 eV, 910 eV, 920 eV, 930 eV, 940 eV, 950 eV, 960 eV, 970 eV, 980 eV, 990 eV, or 1000 eV.

[0060] Using a linear fitting of photocurrents (Figure 5d), the sensitivity values were extracted at several bias voltages. The sensitivity of a detector device with 9 nm thick SnS nanosheets can reach up to at 1 V (Figure 5e).

[0061] This is among the highest sensitivity factors reported to date for soft-X-ray detectors in the water window region (Figure 6), and it is significantly higher than conventional perovskite based detectors.

[0062] Due to the tuneability of the peak sensitivity of the detectors with varying tin mono-chalcogenide thicknesses in accordance with embodiments of the present invention, the peak sensitivity of the tin mono-chalcogenide nanosheet based detectors of the invention at a bias voltage of 1 V may be approximately 0.5x10 ⁴

[0063] Furthermore, the high sensitivity of SnS based soft X ray detectors is also reflected in transient responses to several photon fluxes. This was performed for the 1 .7 nm SnS device, for which the photon flux was controlled by changing the X-ray beam slit size (Figure 4e) at several increments between 120 um (~ 2.4 x 10 ¹² photons/second) and 300 um (~ 6 x 10 ¹² photons/second ). Measurements carried out at a fixed bias voltage of 1 V and photon energy of 600 eV show that the current response increases linearly with increasing slit size, demonstrating the detector’s ability to operate at high photon fluxes with differentiable dynamic output signals at these fluence levels.

[0064] Meanwhile, transient behaviour provides an important measure of an X-ray detectors on/off response characteristics. Upon soft X-ray illumination, the on/off response of the detector device to incident radiation was recorded. As shown in Figure 4f, the photocurrent response to a photon energy of 600 eV shows a fast rise time of ~ 7ms and fall time of ~ 2ms. The response time at other soft X-ray energies was also determined (Figure 7), showing an average rise time of ~ 3.4 ms and a fall time of ~ 2.5 ms. These response times are about five times better than existing CrSiTe ₃ and CsPbBr ₃ soft X-ray detectors. ^[2 ’ ^3]

[0065] Generally, the poor response time for direct X-ray detectors based on polycrystalline films is linked to adverse effects of grain boundaries limiting charge transport and the non-uniform response of the detector due to the presence of large grain sizes. The grain boundaries can create trapping levels within the bandgap and introduce potential barriers between neighbouring grains. However, the single crystalline characteristics of tin mono-chalcogenides prepared by liquid metal based exfoliation methods in accordance with the present invention results in less defective materials with high electron mobilities to enable more effective electron replenishment and lower recombination within the film.

[0066] In accordance with embodiments of the present invention, the on transient response times (rise times) to soft X-rays with photon energies in the range of 100 eV to 1 keV of the tin mono-chalcogenide nanosheet based detectors of the invention may be approximately 2ms, 3ms, 4ms, 5ms, 6ms, or 7ms.

[0067] In accordance with embodiments of the present invention, the off transient response times (fall times) to soft X-rays with photon energies in the range of 100 eV to 1 keV of the tin mono-chalcogenide nanosheet based detectors of the invention may be approximately 1 ms, 2ms, 3ms or 4ms.

[0068] In terms of postulating the operating mechanism of the detectors of the present invention, it is generally considered that when a material is exposed to ionizing radiation, three interactions have actual significance: (i) the ionizing radiation is absorbed by electrons (i.e. photoelectric absorption); (ii) the ionizing radiation undergoes inelastic scattering (i.e. Compton effect); or (iii) the ionizing radiation generates positron-electron pairs (i.e. pair production). Of these three interactions, the photoelectric absorption mechanism dominates for ionizing radiation with energies up to several hundred keV, while the pair production mechanism requires extremely high energy ionizing radiations, mostly in the range of 5 to 10 MeV. The Compton scattering mechanism occurs at intermediate energies between these two extremes.

[0069] Without wishing to be bound by theory, it is therefore considered that when tin mono-chalcogenides are exposed to soft X-rays, it is expected that the photoelectric effect is dominant. In this process, X-rays with energy greater than the electron binding energy, yield the carriers that contribute to electric conductivity. For example, because electrons in shells N2-5 of Sn have electron binding energies of less than 100 eV, they can be ionized using 100 eV soft X-ray photons and further increases in incident soft X- ray photon energy produce secondary electron cascades with high kinetic energies, therefore, creating sufficient charge carrier for efficient detection under external applied bias voltage.

[0070] The X-ray detector characteristics of the tin mono-chalcogenide nanosheets of the present invention represent major improvements in sensitivity and response time compared to existing direct soft X-ray detectors. The sensitivity may be further enhanced and the peak response tunable within the water window region by stacking more tin mono-chalcogenide layers. Importantly, despite the nanosheets being only several nanometers in thickness, the high-absorption coefficients of these materials in the water window region enables excellent and tunable X-ray detection characteristics to be achieved. A further advantage provided by the tin mono-chalcogenide nanosheets of the present invention compared to existing perovskite class materials is their low water solubility, and, in the case of SnS nanosheets, low toxicity, enabling their application to the imaging of living systems in an aqueous environment.

[0071] The tin mono-chalcogenide nanosheet based soft X-ray detectors of the present invention provide significant potential for broad application in a range of soft X-ray detection devices, including, but not limited to sensors, detector modules, detector packages, detector arrays, microscopes, imaging devices, dose rate measurement devices, and real-time dose rate measurement devices. EXAMPLES

Material Synthesis and Fabrication

[0072] Synthesis of tin mono-sulfide (SnS) nanosheets was carried out in a well- controlled glove box environment. The glove box was purged with nitrogen gas supplied at a flow rate of 0.5 seem for approximately 4hrs in order to remove any residual oxygen or other gaseous impurities, followed by the introduction of hydrogen sulfide (H2S) gas at a flow rate of 0.5 seem for approximately 1 hour to achieve a sulfide rich environment. Elemental tin (99.8% Roto Metals) was placed on a glass slide and subsequently melted using a calibrated ceramic heater. The layers on the molten tin surface were detached through pre-conditioning. The liquid metal experiences a colour change from silver to dull yellow within a few seconds indicating the growth of the metal sulfide layer on the surface of molten metal. The SnS nanosheets were then transferred onto a Si/SiC>2 support substrate.

[0073] To avoid thermal shock, the substrate was preheated by placing on the ceramic heater prior to touch printing. An Electron-beam lithography and metal evaporation system (Figure 8) was used to fabricate 100-nm thick gold electrodes on top of the SnS nanosheets with an exposed area of around 800 um ² .

Structural Characterisation

[0074] Atomic force microscopy (AFM) measurements were conducted using a Bruker Dimension Icon AFM in ScanAsyst mode to calculate the thickness of the nanosheets.

[0075] Raman spectra were recorded with a Horiba Scientific Raman spectrometer utilizing a 532 nm laser source operating at 9mW excitation power, 50x objective, and an 1800 gm/mm grating; to characterize the SnS nanosheets.

[0076] X-ray photoelectron spectra (XPS) were recorded by employing a Thermo K- Alpha instrument equipped with a monochromatic Al Ka source (photon energy of 1486.7 eV, pressure of 1 x10 ⁹ Torr, X-ray spot size of 30-400 um). Using a carbon 1 s peak at 284.8 eV, the obtained binding energies were standardized. [0077] UV-v/s absorbance measurements were obtained via a CRAIC 20/30 microspectrophotometer.

[0078] Transmission Electron Microscopy (TEM) was performed with JEOL 1010 and 2100F microscopes. Operated at an accelerating voltage of 100keV, the JEOL 1010 microscope was equipped with a Gatan Orius SC600A CCD camera. The HEOL 21 OOF microscope was equipped with a Gatan Orius SC1000 CCD camera and a Gatan Imaging Filter (GIF).

[0079] Tridiem was utilized to perform electron energy loss spectroscopy (EELS) with an accelerating voltage of 80 keV.

[0080] High-resolution transmission electron microscopy (HRTEM) imaging was performed using the JEOL 21 OOF.

X-ray Detection Measurements

[0081 ] The Australian synchrotron soft X-ray beamline (100 eV to 2500 eV) and Monash X-ray Platform were utilized for soft and hard X-ray detection measurements respectively. Firstly, as a contol experiment, the device without SnS nanosheets was tested to check for any response from the Si/SiO2 substrate. Then the device with SnS nanosheets was exposed to soft X-rays. The device did not show any response above 1.1 keV; therefore, the energy range around 100 eV to 1 keV was utilized to probe the detecting capabilities of the SnS nanosheets. The two-electrode device was placed on a sample holder with insulation tape. The dose rate was recorded using a portable Radiation Monitor Controller (Radcal Corporation Model 9010). The photocurrent was extracted using a Keithley source-meter (2400 series). The l-V (current as a function of voltage) and l-T (current as a function of time) curves were recorded using Quick IV measurement software. All the measurements were carried out at room temperature.

Results

[0082] Raman spectroscopy (Figure 1 a) carried out on the exfoliated nanosheets confirmed the presence of the SnS planar structure through its four characteristic A _g (1 ) (95 cm ^-1 ), Bag (158 cm ^-1 ), A _g (2) (185 cm ^-1 ) and A _g (3) (220 cm ^-1 ) Raman modes.

[0083] A detector device with minimum 1.7 nm thickness of SnS was fabricated and exposed to soft X-rays to extract the photocurrent at several photon energies (100 eV to 1 keV). Figure 3a shows the AFM image of the continuous film with a large area and the corresponding line scan height profile illustrates the thickness of the SnS film which is around ~ 1.7 nm. Considering an SnS monolayer thickness of 0.7 nm ^[4] , this suggests that such nanosheets possess ~3 layers. At an applied bias voltage of 1 V, the detector provided an excellent response at around 600 eV (Figure 3b). The photocurrent increases linearly with increasing photon energies up to 600 eV and decreases smoothly for photon energies higher than 600 eV, with very low response around 1 keV, indicating an excellent response around the water window region. The sensitivity in this thicker device can reach approximately a bias voltage of 1 V.

[0084] A second detector device with minimum thickness of SnS of approximately 9 nm was fabricated (Figure 5a) and exposed to soft X-rays. Figure 3b compares the l-V curves under no illumination and X-ray ON with several photon energies from 100 eV to 1 keV. The l-V curves under several X-ray photon energies shows the maximum signal at around 700 eV. The signal photocurrent increases from 100 eV to 700 eV, followed by a rapid decrease from 800 eV and above. This thicker device demonstrates a peak signal at a photon energy of 700 eV (Figure 5c), indicating a shift in response from 600 eV (for the 1 .7 nm thick device) to 700 eV in the 9 nm thick device. The general trend is similar compared to device 1 (Figure 3d). The drain current at fixed bias voltages was plotted against dose rate (Figure 5d). The slope from this plot was used to calculate the sensitivity values (Figure 5e). The sensitivity in this thicker device can reach approximately 2.55x10 ^{4 u} at a bias voltage of 1 V.

Calculation of Attenuation Coefficient

[0085] The XCOM: Photon Cross Sections Database provided by National Institute of Standards and Technology (NIST), U.S. Department of Commerce (https://www.nist.gov/pml/xcom-photon-cross-sections-databas e) was used to calculate the attenuation coefficient for SnS. Notably, this software can only work in an energy range from 1 keV to 100 GeV. For the present invention, measurements are conducted from 100 eV to 1 keV (within the water window region). The obtained results from the XCOM database (in cm ² /g) may be multiplied to the density of a tin mono-chalcogenide (i.e. 5.22 g/cm ³ for SnS), to calculate the absorption coefficients as a function of photon energy. Calculation of Average Photon Range or Average Penetration Depth

[0086] The attenuation length of a photon effectively describes the distance travelled by a photon before absorption occurs. It is a useful concept to visualize the penetrating characteristics of photons in a material and it is calculated as:

[0087] The average penetration depth is also dependent on photon energy, type of material (atomic number), and material density. Notably, absorption within a material is exponentially dependent on absorption depth. As such, while the attenuation length describes the average depth of absorption, it doesn’t consider the absorption profile within the material. Average penetration depth was plotted as a function of photon energy (keV) as shown in Figure 2a. NIST data is only available from 1 keV; therefore, using a second order approximation, average photon range in photon energies less than 1 keV were be predicted via extrapolation. At 1 keV, around 250 nm of SnS is required to obtain an excellent sensitivity (Figure 1 b). For photon energies lower than 1 keV; the black lines show the extrapolation, which further indicate that few nanometres are sufficient to effectively absorb most of the soft X-rays.

Calculation of Half Value Laver (HVL)

[0088] HVL can be defined as the thickness of material penetrated by one half of the radiation and is generally expressed in units of distance (cm). This is dependent on the penetrating ability of specific radiations and the penetration through thickness of a medium or material. High energy photons can increase HVL values of a material. This parameter can be calculated as;

[0089] Here, the “0.693” shows the exponent value that can give a penetration of 0.5 (ie; e-° ⁶⁹³ = 0.5) Calculation of Attenuation Efficiency

[0090] Softer X-rays (within the energy range from 100 eV to 2 keV) can be absorbed in thin materials as they carry comparatively little energy and can be absorbed in short distances. On the contrary, harder X-rays (2 keV -20 keV) require much thicker materials with increased stopping power as they deposit large amounts of energy. According to the Lambert-Beer-Bouguer law, when the material is irradiated by a beam of photons, the intensity can be expressed as;

[0091] Here « is the absorption coefficient of the material (with thickness x) for specific energies of photons, and is the initial optical intensity. The attenuation efficiency can therefore be written as;

[0092] Using the absorption coefficients of different perovskites and conventional materials at different X-ray photon energies (Figure 2b), attenuation efficiencies were plotted for thicknesses in the range 0.001 um to 100 um for 1 keV to 10 keV photons (In Figure 2c). For comparison several other materials were also included in Figure 2c. We excluded photon energies in decimals as photon energy values in decimals are different for different materials and therefore efficiency estimations as a function of thickness become inaccurate, and selected only the same photon energy values for all materials with the result that the ploted curve is not smooth. The plot in Figure 2c indicates that SnS with high densities 5.22 g/cm ³ provide a better candidate for Soft X-ray photon absorption than existing perovskite class materials.

GENERAL

[0093] Each document, reference, patent application or patent cited in this text is expressly incorporated herein in their entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application or patent cited in this text is not repeated in this text is merely for reasons of conciseness.

[0094] It should be appreciated that throughout this specification, any reference to any prior publication, including prior patent publications and non-patent publications, is not an acknowledgment or admission that any of the material contained within the prior publication referred to was part of the common general knowledge as at the priority date of the application.

[0095] Any manufacturer’s instructions, descriptions, product specifications, and product sheets for any products mentioned herein or in any document incorporated by reference herein, are hereby incorporated herein by reference, and may be employed in the practice of the invention.

[0096] The invention described herein may include one or more range of values (eg. size, displacement and field strength etc). A range of values will be understood to include all values within the range, including the values defining the range, and values adjacent to the range which lead to the same or substantially the same outcome as the values immediately adjacent to that value which defines the boundary to the range.

[0097] The present invention is not to be limited in scope by any of the specific embodiments described herein. These embodiments are intended for the purpose of exemplification only. Functionally equivalent products, processes and methods are clearly within the scope of the invention as described herein.

[0098] Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. The invention includes all such variation and modifications. The invention also includes all of the steps, features, formulations and compounds referred to or indicated in the specification, individually or collectively and any and all combinations or any two or more of the steps or features. REFERENCES

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