Patent Grant
10900115
This application is a national phase entry of PCT/SG2017/050434, filed on Aug. 31, 2017, which claims the benefit of priority of Singapore Patent Application No. 10201607347V, filed on Sep. 2, 2016, the contents of which being hereby incorporated by reference in their entirety for all purposes.
Various aspects of this disclosure relate to a chalcogenide film. Various aspects of this disclosure relate to a device including a chalcogenide film. Various aspects of this disclosure relate to a method of forming a chalcogenide film.
There has been much research related to mid-infrared (mid-IR) radiation as such a region contains fingerprints of the most common molecular vibrations and may also pass relatively unaffected through the atmosphere due to the transparent window in the atmosphere at such wavelengths. Furthermore, mid-infrared radiation has tremendous applications in emerging fields such as optical communications, infrared imaging, and analytical sciences.
Commercial mercury cadmium telluride (HgCdTe), a most widely adopted mid-IR material, exhibits broadband response, which may be achieved by adjusting the alloy composition, or externally changing operation temperatures. Indium antimonide (InSb) is a direct bandgap semiconductor that has been extensively used in short-wave mid-IR applications. The operation regime of InSb can be extended to ˜7 μm via nitrogen doping. Apart from these materials that are based on inter-band transitions, compound semiconductor super lattices with alternate layers of different semiconductors have also been developed in the past two decades for mid-IR optoelectronics based on inter sub-band transitions. However, the abovementioned mid-IR materials may have shortcomings, such as environmental toxicity, high cost, and/or complex fabrication processes.
Two-dimensional (2D) materials have atomic thicknesses, and have revolutionized the fields of material science, chemistry, and physics due to their astonishing electrical and optical properties. For instance, graphene, a two-dimensional material with a layer of carbon atoms arranged in a honeycomb manner, has attracted vast interests in mid-IR applications due to its unique optoelectronic properties, such as broadband absorption, ultrahigh carrier mobility, etc.
However, the low absorption coefficient and short carrier lifetime (in the range of picoseconds) remain major issues for developing high-performance mid-infrared optoelectronic devices. The existing challenges provide the motivation for search of alternative 2D materials with intrinsic narrow bandgaps. Black phosphorene (BP), which was recently discovered to be a 2D layered material, has a dependent bandgap from 0.3 eV˜2.0 eV. Although layered BP can be used for mid-infrared photodetectors and modulators, layered BP only covers up to a wavelength of ˜4.1 μm (0.3 eV), and also suffers from relatively poor environmental stability. On the other hand, 2H-phase transition metal dichalcogenides (TMDC) (MX2, M=Mo, W, X═S, Se) are not suitable for mid-infrared operations because of the relatively large bandgap and low carrier mobility. As a result, most of these developed TMDCs are not appropriate for applications in lower-energy spectral ranges.
Various embodiments may provide a chalcogenide film. The chalcogenide film may include a noble metal chalcogenide material having a formula MCx. M may represent a noble metal. C may represent a chalcogen. x may be any one positive value equal to or more than 1.4 and less than 2. The chalcogenide film may be configured to generate electrons and holes upon light incident on the chalcogenide film.
Various embodiments may provide a device including a chalcogenide film.
Various embodiments may provide a method of forming a chalcogenide film. The method may include forming a noble metal chalcogenide material having a formula MCx. M may represent a noble metal. C may represent a chalcogen. x may be any one positive value equal to or more than 1.4 and less than 2. The chalcogenide film may be configured to generate electrons and holes upon light incident on the chalcogenide film.
The invention will be better understood with reference to the detailed description when considered in conjunction with the non-limiting examples and the accompanying drawings, in which:
The following detailed description refers to the accompanying drawings that show, by way of illustration, specific details and embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, and logical changes may be made without departing from the scope of the invention. The various embodiments are not necessarily mutually exclusive, as some embodiments can be combined with one or more other embodiments to form new embodiments.
Embodiments described in the context of one of the methods or one of the chalcogenide films/devices is analogously valid for the other methods or chalcogenide films/devices. Similarly, embodiments described in the context of a method are analogously valid for a chalcogenide film and/or device, and vice versa.
Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments. Features that are described in the context of an embodiment may correspondingly be applicable to the other embodiments, even if not explicitly described in these other embodiments. Furthermore, additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.
The word “over” used with regard to a deposited material formed “over” a side or surface, may be used herein to mean that the deposited material may be formed “directly on”, e.g. in direct contact with, the implied side or surface. The word “over” used with regard to a deposited material formed “over” a side or surface, may also be used herein to mean that the deposited material may be formed “indirectly on” the implied side or surface with one or more additional layers being arranged between the implied side or surface and the deposited material. In other words, a first layer “over” a second layer may refer to the first layer directly on the second layer, or that the first layer and the second layer are separated by one or more intervening layers.
In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.
In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance.
As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.
In other words, the film 100 may include a noble metal chalcogenide material, which may include a noble metal element and a chalcogen element. The film 100 may exhibit electron-hole generation in the presence of light.
The noble metal chalcogenide material may form a crystal structure. In other words, the film 100 may include one or more crystals of the noble metal chalcogenide material. The film 100 may include crystalline flakes of the noble metal chalcogenide material.
In various embodiments, x may be 2. In other words, the formula may be MC2. In other words, the atomic ratio of the noble metal to the chalcogen is 1:2.
In various embodiments, the crystal structure or the crystalline flakes may include defects or vacancies.
In various embodiments, x may be any one positive value equal to or more than 1.4 and less than 2 (i.e. 1.4≤x<2). In other words, the atomic ratio of the noble metal to the chalcogen may range from 1:1.4 to less than 1:2. When x<2, the film 100 may include vacancies (of the chalcogen) or defects. Increasing the vacancies or defects may lead to a decrease in bandgap, which may lead to an increase in electron-hole generation at a particular wavelength or range of wavelengths, e.g. in the mid-infrared range. In various embodiments, increasing the vacancies or defects may control the bandgap of the film 100.
In various embodiments, x may be any positive value less than 2. In various embodiments, x may be any positive value less than 1.8. In various embodiments, x may be any positive value less than 1.6. In various embodiments, x may be any positive value equal to or more than 1.4.
Noble metal elements may refer to elements such as ruthenium (Ru), rhodium (Rh), palladium (Pd), silver (Ag), osmium (Os), iridium (Jr), platinum (Pt), and/or gold (Au).
Chalcogens may refer to elements such as sulfur (S), selenium (Se), tellurium (Te), and/or polonium (Po). Chalcogenides may include sulfides, selenides, tellurides, and/or polonides.
In various embodiments, the noble metal chalcogenide material may be, but is not limited to, any one material selected from a group consisting of platinum selenide (PtSex), platinum sulfide (PtSx), palladium sulfide (PdSx), and palladium selenide (PdSex).
For avoidance of doubt, noble metal chalcogenide materials as described herein may not include oxides.
In various embodiments, the chalcogenide film 100 may be a two dimensional (2D) film. In various other embodiments, the chalcogenide film 100 may be a three dimensional (3D) film.
In various embodiments, the chalcogenide film 100 may be or may include a single monolayer of the noble metal chalcogenide material. In various other embodiments, the chalcogenide film 100 may be or may include a bilayer of the noble metal chalcogenide material, or a trilayer of the noble metal chalcogenide material. In various embodiments, the chalcogenide film 100 may be or may include a multilayer of the noble metal chalcogenide material. A “multilayer” used in the current context may refer to a film including more than one layer, and may include a bilayer or a trilayer.
In various embodiments, the chalcogenide film 100 may include one or more flakes. Each of the one or more flakes may be a monolayer, a bilayer, a trilayer, or a multilayer. Each monolayer or layer may be a single crystalline sheet of noble metal chalcogenide material. For instance, for platinum selenide (PtSex), each monolayer or layer may include one layer of Pt and two layers of Se such that the Pt layer is between the two Se layers.
Each bilayer may be a stack including or consisting of two crystalline sheets. Each trilayer may be a stack including or consisting of three crystalline sheets. Each multilayer may be a stack including or consisting of a plurality of crystalline sheets. A crystalline sheet used in the current context may refer to a continuous crystal structure. A crystalline sheet may include noble metal atoms and chalcogen atoms bonded strongly to one another. A bilayer or multilayer structure may include two or more crystalline sheets held to neighboring crystalline sheets via van der Waals' forces.
In various embodiments, the light may be or may include visible light, i.e. light having a wavelength or range of wavelengths selected from 400 nm to 700 nm.
In various other embodiments, the light may be or may include infrared light, i.e. light having a wavelength or range of wavelengths selected from 700 nm to 1 mm. In various embodiments, the infrared light may be or may include mid infrared light, i.e. light having a wavelength or range of wavelengths selected from 4 μm to 11 μm.
In various embodiments, the noble metal chalcogenide material may be platinum selenide (PtSex). In various embodiments, the noble metal chalcogenide material may be any one selected from a group consisting of PtSe1.8, PtSe1.6, and PtSe1.4.
Various embodiments may provide a device including the chalcogenide film 100. The device may be an optical device or an optoelectronic device.
In various embodiments, the device may be a photodetector or a solar cell.
In various embodiments, the device may further include a gate electrode (not shown in
In various embodiments, the device may be an optical modulator or any other photonic or optoelectronic device.
In other words, various embodiments may relate to forming a noble metal chalcogenide material, which may include a noble metal element and a chalcogen element.
In various embodiments, x may be 2. In other words, the formula may be MC2. In other words, the atomic ratio of the noble metal to the chalcogen is 1:2.
The noble metal chalcogenide material may form a crystal structure. The film may include crystalline flakes of the noble metal chalcogenide material.
In various embodiments, the crystal structure or the crystalline flakes may include defects or vacancies.
In various embodiments, x may be any one positive value equal to or more than 1.4 and less than 2 (i.e. 1.4≤x<2). In other words, the atomic ratio of the noble metal to the chalcogen may range from 1:1.4 to less than 1:2. When x<2, the film 100 may include vacancies (of the chalcogen) or defects.
Forming the noble metal chalcogenide material may include a chemical vapour transfer (CVT) process, which may be also referred to as chemical vapor deposition (CVD).
Forming the noble metal chalcogenide material may include heating or annealing a noble metal and a chalcogen in the presence of catalysts. In various embodiments, a molar ratio of noble metal:chalcogen of less than 1:2 (e.g. 1:1.8, 1:1.6, 1:1.4 etc.) may be used to form defects/vacancies in the noble metal chalcogenide material.
Forming the noble metal chalcogenide material may include heating a noble metal with a chalcogen in the presence of catalysts.
In various embodiments, the catalysts may include one or more selected from a group consisting of sulfur, phosphorous, bromine and iodine. Sulfur and phosphorous may be used to form PtSex. The formation of PtSex, instead of PtSx or PtSySez (where y+z<2) may be dependent on temperature. The temperature may be very high during synthesis of PtSex. In such a case, the sulfur may be sublimated without reacting with Pt atoms due to the thermodynamic conditions. For example, PtSeS does not exist, as it is thermodynamically unstable. As a result, sulfur may only act as a catalyst in the growth of PtSex crystals. The sulfur sublimate (gas state) may carry the active Se atoms and may provide proper reaction environment (similar substances dissolve mutually). Bromine (Br2) and phosphorous may be used to form PtSx. Iodine and phosphorous may be used to form PdSx and/or PdSex. The use of catalysts may help to facilitate the formation and growth of high quality single crystals.
In various embodiments, the heating or annealing may be carried out in the presence of a transport gas. The transport gas may be iodine, i.e. iodine vapor. Alternatively, the transport gas may be argon.
In various embodiments, one or more single crystals of the noble metal chalcogenide material may be formed using the chemical vapor transfer process. The noble metal and the chalcogen may be heated or annealed in the presence of catalysts using a suitable means, e.g. in a furnace, to form the noble metal chalcogenide material. When the noble metal chalcogenide material is cooled, one or more single crystals of the noble metal chalcogenide material may be formed.
In various embodiments, one or more crystalline flakes may be mechanically exfoliated from the one or more single crystals to form the chalcogenide film. The chalcogenide film may include the one or more crystalline flakes. In other words, forming the noble metal chalcogenide material may include a chemical vapour transfer process to form one or more single crystals. Forming the noble metal chalcogenide material may further include mechanically exfoliating one or more crystalline flakes from the one or more single crystals to form the chalcogenide film. The one or more crystalline flakes may be mechanically exfoliated from the one or more single crystals by using tape.
In various embodiments, the noble metal may be platinum, and the chalcogen may be selenium to form platinum selenide (PtSex).
In various embodiments, a molar ratio of platinum:selenium:phosphorous:sulfur of 1:1.8:1:3 may be used to form PtSe1.8. In various embodiments, a molar ratio of platinum:selenium:phosphorous:sulfur of 1:1.6:1:3 may be used to form PtSe1.6. In various embodiments, a molar ratio of platinum:selenium:phosphorous:sulfur of 1:1.4:1:3 may be used to form PtSe1.4.
Various embodiments may relate to a chalcogenide film formed by a method as described herein.
Various embodiments may relate to a method of synthesizing PtSe2 crystals by chemical vapor deposition (CVD). This may allow the fabrication of PtSe2 field effect transistors (FET) with various numbers of layers. It is observed that trilayer and thicker PtSe2 may be metallic, while bilayer and monolayer PtSe2 may be semiconducting. These observations may show that phase transition in PtSe2 films may be dependent on the number of layers.
Further, it is demonstrated that bilayer PtSe2 may retain a narrow band gap, may possess a relatively high carrier mobility, and may show remarkable photo-response in a broadband region from visible to mid-infrared wavelengths. The results may be of great importance for the future deployment of stable and high-performance infrared cameras, photodetectors, modulators and so on.
The responsivity of the bilayer PtSe2 based photodetector may be two times higher than that of a commercial quantum well InP (QWIP) photodetector. In addition, the detectivity of the bilayer PtSe2 based photodetector may be on par with that of the commercial HgCdTe photodetector and the QWIP photodetector, and may be much higher than that of graphene photodetectors.
In various embodiments, the bandgap of bilayer PtSe2 may be controlled from about 0.32 eV to about 0.16 eV, and may exhibit significant light absorption in the mid infrared range (about 4 μm to about 11 μm).
In various embodiments, a chemical vapor transfer (CVT) method may be used to form high quality, atomic layered noble metal chalcogenides such as sulfides or selenides (i.e. MCx or MC2, wherein C is S or Se).
The molar ratio of platinum (powder, 99.9%, Sigma-Aldrich):selenium (powder, 99.9%, Sigma-Aldrich):red phosphorus (lump, 99.9%, Sigma-Aldrich):sulfur (powder, 99.99%, Sigma-Aldrich) used may be 1:2:1:3 with a total weight of 700 mg. In addition, 35 mg of iodine may be sealed together with platinum, selenium, red phosphorous and sulfur in an evacuated 20 cm long quartz tube 402 under vacuum at 10−6 Torr, which may then be placed in a furnace 404. The furnace may be a three-zone furnace, i.e. the furnace may include three zones in which the material sources may be placed. The three zones may have different temperatures. In other words, the zones may be defined by temperature distribution. As shown in
Large well-formed plates of PtSe2 may be obtained in the growth zone. The crystal structure of PtSe2 may be confirmed by X-ray powder diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX).
In order to form PdSex, Pd powder (99.98%), Se powder (99.94%), red phosphorus and sulfur (from Alfa Aesar), may be thoroughly mixed together in an atomic ratio of Pd:Se:P:S of 1:x:1:3. For instance, an atomic ratio of Pd:Se:P:S of 1:2:1:3 may be used to form PdSe2, and an atomic ratio of Pd:Se:P:S of 1:1.8:1:3 may be used to form PdSe1.8. The mixture may then be loaded, and sealed in an evacuated quartz ampule, slowly heated up to about 900° C., and held for 50 h. The mixture may then be slowly cooled to 450° C. at a rate of 3° C./h to room temperature.
Precise control of the x value may theoretically be difficult due to the limit of characterization at the atomic level in a large-area device. Raman spectroscopy and other non-destructive methods may be used to monitor the value of x. The temperature setting strategy described herein may enable a skilled person to reproduce a noble metal chalcogenide material according to various embodiments. The temperature may be tuned to adjust the value of x. The material may then be characterized. The band gap may then be controlled by controlling the concentration of vacancies as shown by the simulation results provided herein.
The strong, narrow peaks for (001), (012) and (111) in the XRD may indicate the high crystal quality of the synthesized samples. PtSe2 may crystallize in the 1T-type layered structure in the trigonal space group P
λ=2d sin(θ) (1)
where n is a positive integer and λ is the wavelength of the incident X-ray light. As a result, the lattice distance (d) of the (001) facet may be 2.56 Å based on the diffraction angle θ of ˜18°.
In order to form vacancies or defects in the crystalline PtSe2, the Se may be reduced relative to the Pt used. A similar set up to that shown in
After forming the single crystals, layers or flakes of platinum selenide may be exfoliated and transferred to a copper grid.
The XRD may be similar to that shown in
The quality of PtSe2 sample may be further characterized by transmission electron microscopy (TEM) in order to clarify its atomic structure.
The TEM, combined with the XRD pattern may demonstrate the successful growth of highly crystalline platinum selenide. The obtained platinum selenide may possess layered 1T-type hexagonal crystal structure with space group P-3m1 and lattice parameters, a=b=3.7278 Å and c=5.0813 Å.
A layer of PtSe2, which may be referred to as a crystalline sheet, may consist of three layers of atoms stacked in the order of Se—Pt—Se. The different layers or crystalline sheets of PtSe2 in a multilayer stack may be held together by weak van der Waals forces. Within a single PtSe2 layer or crystalline sheet, the Se atoms may be strongly bonded with the Pt atoms to form octahedral prismatic local coordination structures, wherein each coordination structure includes a Pt atom at the center of each coordination structure. The PtSe2 octahedral structures within the single PtSe2 layer or crystalline sheet may be connected to one another by sharing Se—Se edges. The layered crystal structure of PtSe2 may be analogous to traditional TMDCs. Monolayers, bilayers, or multilayers of PtSe2 may be formed by proper exfoliation strategies.
PtSe2 flakes or atomic layers may be exfoliated from the bulk crystals by Scotch tape and transferred onto a silicon wafer with a 285 nm thermal oxidation layer. The flakes may, for instance, consist of a single crystalline sheet (monolayer), two crystalline sheets (bilayer), or three crystalline sheets (trilayer). The optical images of monolayer, bilayer and trilayer PtSe2 on the SiO2/Si wafer are shown in
The thicknesses of the samples were determined by the AFM height profiles measured along the dashed lines crossing the flakes. The thickness of PtSe2 monolayer may be about 0.63 nm as shown in
The layer-dependent properties may also be characterized by Raman spectroscopy similar to other two-dimensional material characterizations.
On the other hand, the shear mode frequencies (i.e. frequencies less than or equal to about 50 cm−1) may indicate the layered structure of platinum selenide flakes. The shear mode frequencies may be used to accurately determine the number of layers in platinum selenide flakes.
In order to understand the dependency of band structure and bandgap of platinum selenide (PtSe2) on the number of layers, first-principles calculations may be performed. From
The small bandgap of bilayer platinum selenide may make it suitable for light absorption in the mid-infrared (λ>4 μm) region. Bilayer platinum selenide may be a suitable candidate to fill in the gap between graphene and commonly employed TMDCs, and may be a suitable two dimensional (2D) semiconductor candidate material for mid-infrared photonic and optoelectronic applications.
Platinum selenide field effect transistors (FETs) may be fabricated to investigate mid-infrared photo-detection, thereby demonstrating the optoelectronic properties of atomic layered platinum selenide.
The photodetector may include a heavily doped silicon substrate and a chalcogenide film including the platinum selenide flakes on the substrate. The platinum selenide flakes may be atomic-layered, graphene-like or graphite-like flakes, i.e. containing one or more crystalline sheets, and may be mechanically exfoliated from the CVT synthesized platinum selenide single crystals using adhesive 3M-tape and deposited on a silicon wafer with a 285-nm thermalized silicon oxide (SiO2) layer. The location and quality of atomic-layered platinum selenide flakes may be identified or determined by optical contrast using an optical microscope and Raman spectroscopy.
The photodetector may also include a first electrical contact and a second electrical contact in contact with the chalcogenide film. The first electrical contact may be spaced apart from the second electrical contact. The electrical contacts may be formed using electron beam evaporation after standard lithography procedure, and may each include 20 nm of titanium (Ti) and 80 nm of gold (Au) on the titanium. The heavily doped silicon substrate may be used as a back gate. The first electrical contact may be taken as the drain and the second electrical contact may be taken as the source.
Electrical characteristics such as drain/channel current−drain voltage (ID−VD) and drain current−gate voltage (ID−VG) curves may be measured by a semiconductor analyzer (Agilent, B1500 Å). All measurements are carried out in the dark under room temperatures. The source electrode may be connected to ground.
The mobility (μ) of the carriers may be calculated by:
where L denotes the channel length, W denotes the channel width, and d denotes the thickness of the silicon oxide (SiO2) layer. The thickness of the silicon oxide layer is about 285 nm in the experiments. In addition, Vds denotes the source-drain bias, Ids denotes the channel current, and Vb denotes the bottom gate voltage when the device is operating in the linear region in the Id/Vg curve. ε0 is the vacuum dielectric constant, and εr is the dielectric constant, which is 3.9 for silicon oxide. The calculated carrier mobility of monolayer platinum selenide (PtSe2) based FET is about ˜1.7 cm2/V s.
The electrical measurements indicate that trilayer platinum selenide may be metallic and may show negligible modulation by the gate voltage. Further, the metallic behavior of trilayer platinum selenide may indicate that it is unsuitable for use in photo-detection.
The metallic behavior of trilayer platinum selenide, as well as the semiconducting behavior of monolayer and bilayer platinum selenide may be consistent with theoretical predictions of epitaxy grown platinum selenide on platinum substrate. The measurements also show that monolayer and bilayer platinum selenide may exhibit n-type semiconducting behavior with an on/off ratio in the range of about 20 to about 50. The calculated carrier mobilities of monolayer and bilayer platinum selenide may be on par with previous reports on InSe, In2Se3 etc. The lower carrier mobility of monolayer platinum selenide relative to bilayer platinum selenide may be attributed to the surrounding conditions or the charge transfer from neighboring adsorbates and the substrate.
The photo-responsivity measurements may be performed in a digital deep level transient spectroscopy system (BIORAD) with visible, near-infrared and mid-infrared lasers. All samples used in the photo-responsivity measurements have dimensions of ˜3.5 μm×10 μm.
As discussed above, monolayer platinum selenide may be a semiconductor with an indirect bandgap of ˜1.2 eV, which may be suitable for photo detection in the visible and near-infrared region.
The photocurrent (Iph) is defined as:
Iph=Iillium−Idark (3)
where Idark is the dark current and Iillium is the current measured.
As shown in
The power density of all laser illuminations is kept at about 0.25 W/cm2 for time-resolved measurements shown in
The bilayer platinum selenide based FET may exhibit a much stronger or higher photo-response compared with the monolayer platinum selenide based FET. For instance, the photo-responsivities (R) in the visible and near infrared may be about 6.25 A/W and 5.5 A/W respectively. A strong photo response may be observed in the mid-infrared (λ=10 μm) as shown in
Furthermore, the decay time can be fitted by the following equations:
wherein τ is the time constant, and t is the time when laser is switched on or off. A and B are scaling constants. I0 is the initial current.
The fitted characteristic photo-response time constant τ7 is 1.2±0.1 ms for rise time, and time constant τ2, is 1.2±0.1 ms for fall time. The time constants are better than those of other types of 2D semiconducting photodetectors such as MoS2 based photodetectors and black phosphorene based photodetectors.
The rise time and fall time may also be based on the time period between the 10% and 90% of the current in the rising curve and falling curve respectively. The rising curves and falling curves of the monolayer and bilayer based field effect transistors are shown in
The detectivity (D) of a photodetector may be a measure of the sensitivity of the detector. Assuming that the shot noise from dark current and environment is the dominant contribution to the intrinsic noise, the detectivity D can be calculated by:
D=R√{square root over (A)}/√{square root over (2eID)} (6)
wherein R is the photo-responsivity, A is the area of the detector, ID is the dark current as shown in
Further, as shown in
IPC=CPγ (7)
wherein C is a constant and P is the illumination power. For monolayer PtSe2 based FET, γ is 0.90 for a laser at an operating wavelength of 632 nm, and γ is 0.75 for a laser at an operating wavelength of 1.47 μm, highlighting that the recombination kinetics of photo-carriers involve both traps/defects states and photo-generated carrier interactions.
For bilayer platinum selenide FET, the values of γ are 1.16, 1.1 and 0.92 for 632 nm, 1.47 μm and 10 μm respectively. The relatively higher values for bilayer platinum selenide FET samples may be attributed to less trapping and substrate effects compared to monolayer samples. Further investigations may be required to interpret the complex carrier recombination and scattering processes of the photo-generated electron/hole pairs. Furthermore, the increase of the photocurrent with the incident laser power, as shown in
It is also noted that the response speeds of the bilayer platinum selenide FET may be quite fast, e.g. in the range of milliseconds, as compared to the previously reported results of graphene nanostructured photodetectors and 2D semiconducting photodetectors. However, the mid-infrared photo-detection performance may be further improved by dielectric engineering and surface engineering strategies. For example, the mobility of the platinum selenide FET may be enhanced in the high-dielectric environment due to the reduction of Coulomb interactions among the carriers in the channel. On the other hand, the slow response speed may also be attributed to the trapping states, which may be modified by surface engineering methods, such as chemical treatment and/or trapping molecular/film decorations.
Various embodiments may show great potential in other mid-infrared photonic applications or devices such as mid-infrared modulators, light harvesters, and so on. Besides PtSe2, other noble metal dichalcogenides such as PtS2, PdS2, PdSe2 etc. may be also promising for mid-infrared photonic/optoelectronic devices.
Various embodiments may relate to use of atomic layered PtSe2 as a high performance mid-infrared photodetector that may be operated at room temperature. The high and broadband photo-response of bilayer platinum selenide based FET may be attributed to the decrease in bandgap due to defects.
Experiments have been carried out to study the optical properties and bandgap of the fabricated sample. Exfoliated bilayer platinum selenide flakes (Sample #1) may be transferred to a calcium fluoride (CaF2) substrate and the absorption spectrum of the PtSe2 flakes may be obtained.
The quality and stoichiometric ratio between Pt to Se may be further monitored by X-ray photoelectron spectroscopy (XPS).
The two dominant peaks in the Se 3d spectrum (55.40 eV and 54.50 eV) may exhibit the dominance of Se2-peaks and full crystallization of PtSe2, which are slightly higher than the bonding energies of Se2-atoms and may be explained by the change in the chemical state of Se atoms in the presence of Se vacancies (VSe). The atomic ratio may be calculated by semi-quantitative analysis through measuring the peak areas of Se and Pt core lines (I), and applying the appropriate atomic sensitivity factors of both elements (S) which are known as the relative sensitivity factors (RSF):
Cx=IxSx/ΣIiSi (8)
where Cx is the atomic fraction of element x in the sample. From the calculations, an atomic ratio of Se:Pt of 64:36 may be obtained, which agrees well with the HRTEM images as shown in
In order to examine the role of defects in the electronic structure of platinum selenide, another sample with lower defect concentration may be fabricated (Sample #2) by decreasing the deposition temperature. The atomic ratio of Se:Pt in sample 2 may be estimated to be ˜66.5/33.5. The defect concentration may be verified by XPS spectrum.
From the absorption spectrum, it is clearly shown that the light absorption cut-off of sample #2 is ˜8 corresponding to a bandgap of ˜0.155 eV. The above observations may demonstrate the importance of defect states in bilayer PtSe2. Unlike most TMDCs where the bandgap is enlarged by the defect induced recombination/trapping centers, the bandgap of various embodiments may become narrower with the introduction of Se vacancies.
As a result, it may be important to investigate the defect states in narrow bandgap PtSe2 atomic layers since it may further lead to operations involving longer wavelength operations.
During the synthesis procedure, high temperatures exceeding the melting temperature of Pt may be required to break the Pt—Se bonds, which may cause formation of Se vacancies. After careful optimization of the growth conditions, a technique to control the defect concentration may be developed. The quality of a noble metal chalcogenide such as platinum selenide may be controlled by the evaporation temperature (T1) and deposition temperature (T2).
Alternatively, reduced selenium relative to platinum may be used as starting materials to increase the concentration of selenium vacancies. Single crystals of PtSe1.8 may be prepared by a chemical vapor transport method as described herein, but with a starting ratio of Pt:Se:P:S of 1:1.8:1:3. Similarly, single crystals of PtSe1.6 may be prepared with a starting ratio of Pt:Se:P:S of 1:1.6:1:3, and single crystals of PtSe1.4 may be prepared with a starting ratio of Pt:Se:P:S of 1:1.4:1:3. The other reaction parameters (e.g. temperature, reaction time, pressure etc.) may be the same or similar to the parameters used to form PtSe2, and as illustrated in
In general, the stoichiometry of PtSex may be controlled by varying the Se molar ratio. The value of x may be the same as the molar equivalents of Se used. For instance, if 1.6 equivalents of Se is used, x is 1.6 (i.e. PtSe1.6); and if 1.8 equivalents of Se is used, x is 1.8 (i.e. PtSe1.8). The smallest value of x may be 1.4.
The fabrication of other noble metal chalcogenide crystals may also be possible using similar methods.
The bandgap evolution of bilayer PtSe2 with different concentrations of Se vacancies may be investigated using first-principles calculations.
The valence band maximum in bilayer platinum selenide may be contributed mainly by 5d orbitals of Pt atoms that contain rich d-electrons. As a result, the interaction of the d band of Pt atom and the pz band of Se atom thermodynamically favors sp3d2 hybridization (1T phase), in which less d orbitals are involved and the bond between Pt and Se are weaker than these of group-5 or group-6 TMDCs (such as MoS2, WS2 with spd4 hybridization in 2H phase).
From the density of states (DOS) of bilayer platinum selenide with defects, the mid-gap states due to Se vacancies may be in the vicinity of the Fermi level and may thus decrease the bandgap of bilayer platinum selenide.
The bandgap of intrinsic bilayer platinum selenide may be ˜0.323 eV. However, the bandgap may decrease to 0.082 eV by introducing 6% Se vacancies, which is consistent with the first-principles calculations.
The results show that the bandgap decreases with increasing Se vacancies while remaining indirect in nature for all concentrations simulated. These features indicate the importance of Se vacancies on the bandgap of atomic-layered platinum selenide, i.e. bilayer platinum selenide as demonstrated in the simulation results in
Various embodiments may relate to a two-dimensional platinum selenide crystal or film, which may be metallic or semiconducting. The crystal or film may include large area atomic layers or crystalline sheets. Trilayer and thicker platinum selenide may be semi-metallic, while bilayer and monolayer platinum selenide may be n-type semiconductors with indirect bandgaps (˜0.1 eV for bilayer platinum selenide) and relatively high carrier mobility.
It has also been shown experimentally that the bandgap may be engineered through defect engineering (e.g. by varying Se vacancies) to cover the mid-IR band, which may be important for mid-infrared photonic and optoelectronic applications. The photo-detection properties of monolayer and bilayer platinum selenide based FET devices have been obtained.
It has been found that monolayer platinum selenide based devices may be suitable for visible and near infrared photodetectors, and bilayer platinum selenide based FET devices may be suitable for broadband mid-infrared photodetectors, with broadband response ranging from visible to mid-infrared, along with high responsivity and fast response speed. These results show that platinum selenide and other noble metal chalcogenides or narrow bandgap transition metal chalocogenides (TMCs) may be suitable for optoelectronic applications, e.g. in the mid-infrared regime.
Various embodiments may be alternatives to other 2D materials for photonic and optoelectronic applications.
Computational Details
Electronic structures are calculated within the density functional theory (DFT) framework with the projector augmented wave (PAW) basis using the VASP (Vienna Ab Initio Simulation Package) codes. The Perdew-Burke-Ernzerhof-type generalized gradient approximation (GGA) is used to describe the exchange-correlation energy. The spin-orbit coupling (SOC) is included take into account the relativistic effects. To model different atomic layers structure of PtSe2, a slab model with a vacuum gap of 15 Å between periodically repeated slabs is employed to avoid interaction between the slabs. A plane wave cutoff energy of 500 eV is used. The Brillouin zone sampling is done by using 12×12×12 and 12×12×1 Gamma-centered k-meshes for bulk and slab model respectively. The defect calculations are performed using the supercell geometry of the corresponding unit cell. The total energies in the calculations are converged to 1×10−6 eV and the atomic positions are relaxed until the residual forces on each atom are less than 1×10−3 eV/Å.
Raman Spectroscopy
The layer-dependent properties may also be characterized by Raman spectroscopy similar to other two-dimensional materials. The synthesized bulk PtSe2 and atomic layers show two main Raman peaks near 200 cm−1 and 300 cm−1, which may be defined as Eg mode vibration and Alg mode vibration respectively. The soft mode near 320 cm−1 may be caused by the substrate. The Eg vibration mode and Alg vibration mode change drastically with the decrease in the number of the layers of PtSe2. The variation of the peak intensity ratio may be employed as a signature to distinguish the number of PtSe2 layers. However, from the fingerprint of the Raman spectrum in
Ultralow-frequency (ULF) Raman spectroscopy has been widely used for the characterization of two-dimensional materials, which is extremely sensitive to the number of layer and the layer configurations. As clearly shown in
where ω0 is the bulk shear mode frequency. By fitting the ULF Raman spectroscopy, ω0=32 cm−1 may be obtained for bulk PtSe2.
While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10201607347V | Sep 2016 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2017/050434 | 8/31/2017 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2018/044237 | 3/8/2018 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4242374 | Sansregret | Dec 1980 | A |
| 20010054375 | Fujiwara | Dec 2001 | A1 |
| Number | Date | Country |
|---|---|---|
| WO 2014150635 | Sep 2014 | WO |
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| Number | Date | Country | |
|---|---|---|---|
| 20190194797 A1 | Jun 2019 | US |
