Triply degenerate semimetal PtBi2 as van der Waals contact interlayer in two-dimensional transistor

The low-energy electronic excitations in topological semimetal yield a plethora of a range of novel physical properties. As a relatively scarce branch, the research of triple-degenerate semi-metal is mostly confined to the stage of physical properties and theoretical analysis, there are still challenges in its practical application. This research showcases the first application of the triply degenerate semimetal PtBi2 in electronic devices. Leveraging a van der Waals transfer method, PtBi2 flakes were used as interlayer contacts for metal electrodes and WS2 in transistors. The transistor achieved a switching ratio above 106 and average mobility can reach 85 cm2V−1 s−1, meeting integrated circuit requirements. Notably, the excellent air stability of PtBi2 simplifies the device preparation process and provides more stable device performance. Transfer process reduces the Schottky barrier between metal electrodes and semiconductors while avoiding Fermi pinning during metal deposition to achieve excellent contact. This groundbreaking work demonstrates the practical applicability of PtBi2 in the field of electronic devices while opening new avenues for the integration of novel materials in semiconductor technology, setting a precedent for future innovations.

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Introduction
Topological semimetals, encompassing Dirac [1][2][3][4], Weyl [5,6], nodal-line [7], and triple-degenerate varieties [8,9], are pivotal both from a scientific standpoint and for the advancement of electronic devices that prioritize high functionality, integration, and energy efficiency [10][11][12].The trigonal crystal system of PtBi 2 , a two-dimensional (2D) layered semimetal bonded by van der Waals (vdW) forces, exhibits compelling carrier characteristics that may be attributed to dimensionalityinduced variations in Fermi levels.Recent explorations have shed light on the distinctive linear band structure of PtBi 2 with triple-degeneracy, positioning it as a potential candidate in the realm of triple-degenerate semimetals [13].The triply degenerate point fermions in PtBi 2 are close to the Fermi level.It is noteworthy for its stability in atmospheric conditions and high mobility, reaching up to 2.5 × 10 4 cm 2 V −1 s −1 at low temperatures [13,14].While significant strides have been made in understanding the condensed matter physics of PtBi 2 , including Rashba-like spin splitting [15] and quantum oscillations in magnetoresistance [16], its practical integration into electronic applications remains largely uncharted.
One promising avenue involves the use of semimetals as intermediary layers between 2D transition metal dichalcogenides (TMDs) channels and metal electrodes in transistor designs [17][18][19][20].This approach is advantageous due to the adjustable work function, which can diminish the Schottky barrier at the contact point [21][22][23][24].However, current exploration is nascent and only sputtering single elemental semimetals [17,18] and Dirac semimetal ZrTe 2 [19] have been reported.In general, conventional sputtering processes often induce a Fermi pinning effect, leading to suboptimal device performance [25][26][27].Hence, a potential solution lies in the nondestructive vdW transfer of metal electrodes or stratified materials [28][29][30][31].Transferred ZrTe 2 nanoflakes open this way but the device fabrication process requires preparation and packaging in an argon environment to maintain its integrity, due to the oxygen sensitivity of ZrTe 2 nanoflakes.The use of strippable PtBi 2 semimetal may also circumvent the limitations encountered in sputtering, potentially enhancing electrode contacts in transistors and elevating overall performance.Most importantly, PtBi 2 nanoflakes has oxidation passivity and can preserve physical characteristics for even months, being a powerful candidate for long-term stable devices.To fully harness the potential of PtBi 2 , comprehensive investigations are required based on its work function, Fermi surface properties and electric transport properties.
This work presents the incorporation of the tripledegenerate semimetal PtBi 2 as an intermediary layer in WS 2based transistors, significantly reducing the Schottky barrier in metal−semiconductor (MS) contacts.PtBi 2 flakes are found to exhibit exceptional stability and the preparation of the PtBi 2based device can be carried out under normal conditions, demonstrating the potential for the long-term stable transistor.With vdW contact between materials and metallic electrodes, Fermi pinning effect could be avoided and a linear output is demonstrated.Notably, a switching ratio above 10 6 and a high average mobility of 85 cm 2 V −1 s −1 are achieved, which satisfies the rigorous demands of integrated circuit applications.This advancement in utilizing PtBi 2 not only expands the application spectrum of semimetals in transistor technology but also bridges the gap between theoretical understanding and practical utility, underscoring the potential of PtBi 2 in the field of electronics.

Material preparation
The bulk PtBi 2 crystals were synthesized by the self-flux approach with bismuth (Bi) as the flux agent.The raw materials (Platinum particle: 99.999%, Sigma Aldrich; Bismuth particle: 99.999%, Aladdin) with a stoichiometric ratio of Pt: Bi = 1:8 was mixed and stored in an alumina crucible, and then the crucible was sealed into a quartz tube under vacuum (∼10 −5 Torr).Quartz tube was heated up to 800 • C, dwelled for 24 h, and then slowly cooled to 430 • C with a rate of 2 • C h −1 in a furnace.The tube was taken out from the furnace at this temperature and centrifuged immediately to separate excessive flux.Finally, crystal flakes with metallic luster and sizes larger than 5 mm were obtained.Transition metal chalcogenides (WS 2 , WSe 2 and MoS 2 ) single crystal were purchased from HQ graphene.

Characterizations
The quality of growth PtBi 2 crystal was determined by powder x-ray diffraction (XRD, D8 ADVANCE, Brucker) with Cu Kα radiation (wavelength = 0.154 nm) and Raman spectroscopy (LabRAM HR800, Horiba Jobin-Yvon) with an excitation wavelength of 532 nm.Characterization of a cross-section of the transistor is performed by Field Emission Scanning Electron Microscope (Nova NanoSEM 450) accompany with energy disperse spectroscopy (EDS).The thickness of each layer was measured by atomic force microscopy while the work function of Au, PtBi 2 and WS 2 with different thicknesses were measured by Kelvin probe force microscope (KPFM).

Device fabrication and electrical measurement
PtBi 2 and TMDs flakes, mechanically exfoliated from their respective bulk crystals using blue tape, were initially transferred onto a polydimethylsiloxane (PDMS) sheet.In the subsequent vdW transfer process, these flakes were picked up from the PDMS with the aid of a polypropylene carbonate (PPC) thin film, and then methodically layered onto a SiO 2 /Si substrate.The TMDs flakes served as the channel material, while the PtBi 2 flakes functioned as the source-drain contact interlayer, thus forming a TMDs transistor with a fully vdW contact interface.The final step involved transferring two Au electrodes onto the PtBi 2 flakes through the same vdW transfer process, facilitating measurement.All electrical characterizations of these transistors were performed in a probe station, utilizing a Keysight B1500A semiconductor parameter analyzer.

Results
The establishment of MS contacts is critical for optimizing device performance.Conventional deposition methods often introduce chemical disorders and defects at the interface, negatively impacting the quality of contacts.Consequently, a nondestructive vdW technique is increasingly favored for fabricating PtBi 2 -TMDs contacts.For this purpose, PtBi 2 nanoflakes were mechanically exfoliated from a grown bulk crystal, and vdW contacts with WS 2 were formed through a physical stacking process, aided by PPC (device fabrication process in the Supporting Information figure S1).To ascertain the quality and crystalline phase of the synthesized PtBi 2 , variable temperature resistivity and XRD analyses of the single crystal were conducted, as depicted in figures 1(a) and (b), respectively.The temperature-depend resistivity (R-T curve) of PtBi 2 exhibits pronounced metallic behavior, and the ratio of roomtemperature resistivity to low-temperature residual resistivity (RRR) is exceptionally high (R 300K /R 2K = 145), confirming the high quality of the grown crystal [13].The fitting curve ρ = ρ 0 + αT 2 employed to model the data collected below 50 K, as illustrated in the inset, yielding a residual resistivity ρ 0 ≈ 4.3 µΩ cm.The XRD spectra display sharp peaks along the (0 0 l) plane, indicating high crystallinity and an exposed ab surface.Experimentally, potential band variations are typically reflected in the Raman spectrum.Figure 1(c) presents the Raman characterization of PtBi 2 , WS 2 , and their heterojunction.Unlike previous studies of elemental semimetals (Bi and Sb) [17,18] that exhibit band hybridization with MoS 2 and a consequent shift in Raman peaks, no significant peak shift is observed in the PtBi 2 -WS 2 contact.Triple-degenerate in PtBi 2 as a novel electronic state, is protected by the symmetry of the crystal.Computational results indicate that, with the inclusion of spin-orbit coupling, the spin degenerate and non-degenerate bands do not hybridize upon crossing due to this symmetry protection [8].Given the unique band structure of PtBi 2 , it does not exhibit band hybridization with TMDs.This distinctive characteristic stems from the specific electronic configuration and interactions within PtBi 2 , which fundamentally differ from those observed in other semimetals.Such a property is crucial for its application in semiconductor technology, as it implies a stable and predictable interaction with TMDs, enhancing the potential for high-performance and reliable device integration.
Material stability is a crucial factor in selecting materials for transistor construction [32,33].PtBi 2 , a binary semimetal comprising inert metallic elements, exhibits exceptional stability under normal atmospheric conditions and standard temperature and pressure.The electrical and optical performance of several devices was extensively evaluated over extended periods, as depicted in figures 1(d)-(f).A representative voltampere characteristic of a PtBi 2 -based device demonstrates consistent linearity and resistance even after five months, underscoring its reliability.Such stability in linearity and low resistance are fundamental for semimetal contacts.Although the resistance of PtBi 2 flakes was measured using a two-wire method, which may not precisely represent the true resistivity, four devices with similar PtBi 2 thickness exhibited a consistent resistance profile (around 25 Ω), with only a minimal average increase of 3.88% in resistance over five months.Concurrently, Raman spectroscopy of the PtBi 2 flakes revealed spectra nearly identical to the original, with a negligible shift in wavenumber.These findings collectively affirm the excellent air stability of PtBi 2 , positioning it as a viable candidate for long-term stable transistor contact materials.
The PPC-assisted vdW transfer method provides a pristine, fold-free interface of PtBi 2 -WS 2 and PtBi 2 -Au contact, which circumvents potential damage to the WS 2 contact area occurred during conventional lithography and metal deposition processes [34].This vdW transfer electrodes technique has already been employed which can avoid Fermi pinning from metallic deposition, underscoring its potential for scalable application [35].The cleanliness and integrity of this interface were verified using a scanning electron microscope (SEM), which confirmed a clear cross-section of the transistor (refer to figure S2).Additionally, elemental ratio mapping from EDS spectra corroborated the stoichiometric precision of PtBi 2 .Figures 2(a) and (b) display a schematic diagram of the device structure and an optical image of WS 2 transistors with PtBi 2 vdW contacts, respectively.For these devices, PtBi 2 nanoflakes of nearly uniform thickness were selected and transferred onto the WS 2 surface, with the process repeated twice.The channel length and width measure approximately 17 and 21 µm, respectively.
The transport properties of the WS 2 transistor with the PtBi 2 vdW contact were systematically investigated at both room and low temperatures to assess the impact of PtBi 2 .Figure 2(c) demonstrates a typical output curve (I ds vs. V ds ) at room temperature, where the gate voltage incrementally increases in steps of 10 V. Here, I ds and V ds denote the drain-tosource current and voltage, respectively.The observed unsaturated output with pronounced linearity suggests an efficient contact between the PtBi 2 -WS 2 and PtBi 2 -Au electrodes.In this scenario, the mobility of the transistor was determined and calculated at a fixed V ds of 1 V.The transfer curve, represented by I ds versus V gs (with V gs representing gate-to-source voltage), for V ds values ranging from 0.01 to 1 V, is depicted in figure 2(d).This curve reveals n-type channel conduction with an on/off current ratio (I ON /I OFF ) exceeding 10 6 under room temperature in a vacuum environment.The peak mobility, measured at 72.2 cm 2 V −1 s −1 was derived from the slope of the tangent of the transfer curve at V ds = 1 V, following the equation provided: LdI ds C ox wV ds dV gs where L and w are the length and width of the channel, respectively; V ds and I ds represent source-drain voltage and current respectively C ox is the gate capacitance, V gs is gate voltage.Although this mobility is marginally lower than that achieved with specialized approaches such as tunnelling or edge contacts, it surpasses the performance of transistors with directly deposited metal electrode contacts and does not require complex processing.This level of performance is adequate for integration into transistor applications, demonstrating the efficacy and potential of the PtBi 2 vdW contact approach in semiconductor technology.
Temperature-dependent transport characterization is essential for understanding the behavior of transistors and facilitates the analysis of the contact mode between PtBi 2 and WS 2 .The transfer curves, measured from 300 K down to 110 K (shown in figure 3(a)), display consistent variations in the on-state current.It was observed that the Ids increase with decreasing temperature when V gs > 20 V, indicative of low contact resistance between PtBi 2 and WS 2 .A notable crossover in the range of −30 V < V gs < 20 V is attributed to the metal-insulator phase transition of WS 2 .Remarkably, the gate voltage range for the insulating regime is considerably narrower than that of an intrinsic WS 2 transistor, suggesting an altered transport mechanism [36].Several factors contribute to these transport characteristics.Firstly, at lower temperatures, carrier mobility within the transistor typically increases due to diminished lattice vibrations, thereby reducing scattering.Additionally, lower temperatures may augment the carrier concentration in the semiconductor.Furthermore, the specific contact mode plays a crucial role in defining the overall transistor behavior.For instance, the Schottky barrier height may increase at lower temperatures.Collectively, these factors contribute to an elevation in the source leakage current at reduced temperatures [37,38].To ascertain the presence of a Schottky barrier in the PtBi 2 -WS 2 contact, Arrhenius plots (ln(I DS /T 1.5 ) vs. 1000/T) are further extracted from the temperature-dependent transport results as shown in figure 3(b).Generally, in Schottky barrier devices, the inverse subthreshold slope is influenced by thermally assisted tunnelling.The tunnelling current becomes negligible only when the gate voltage drops below the flat band voltage (VFB).Identifying VFB is crucial for determining the actual Schottky barrier height (Φ SB ) via the Arrhenius plot at this specific gate voltage.This determination involves analyzing the slope of the curves in the high-temperature range, based on conventional thermionic emission theory.Specifically, the slope in the Arrhenius plots can be used to calculate the effective energy barrier (Φ B ) at a given V gs , by employing the simplified equation: where k B is Boltzmann's constant, T is temperature and c is a constant.The simplified process is in Supporting may present a challenge for transistor applications, particularly at low temperatures, it is noteworthy that this barrier is still lower than those typically observed in direct metal electrode contacts to TMD semiconductors [26,39].Intrinsic field-effect mobilities as a function of temperature have also been deduced from the variable-temperature measurements of this two-terminal device, as depicted in figure 3(d).The mobility exhibited a characteristic phonon-limited pattern (µ ≈ T −γ , where γ represents the characteristic exponent of phonon-limited transport and T is the temperature) at elevated temperatures, with γ equating to 1.581 at high temperature.In contrast, at lower temperatures, the mobility transitions to a defect-limited saturation trend, characterized by a change in the exponent to 0.787, attributable primarily to acoustic phonon scattering.This nuanced understanding of the temperature-dependent mobility characteristics is crucial for optimizing the performance of transistors incorporating PtBi 2 -WS 2 contacts [22].The distinctive crossover in the transfer curve of PtBi 2 -WS 2 transistors across various temperatures, can be comprehensively understood through the interplay of several key factors.Firstly, a metal-insulator transition in the WS 2 channel becomes more pronounced with decreasing temperature, altering its electronic properties.This is further influenced by the PtBi 2 -WS 2 contact, where unique semimetal nature of PtBi 2 modifies the electronic characteristics of WS 2 layer.Additionally, the low contact resistance between PtBi 2 and WS 2 enhances charge carrier injection, especially at lower temperatures, due to reduced phonon scattering.This is coupled with the temperature-dependent carrier mobility in WS 2 , which improves with decreasing temperature, facilitated by the near-ohmic PtBi 2 -WS 2 contact.Lastly, the modulation of the Schottky barrier at the PtBi 2 -WS 2 interface under varying temperatures also contributes to the observed transfer curve crossover.Together, these factors synergistically influence the temperature-dependent behavior of PtBi 2 -WS 2 transistors, highlighting the intricate relationship between material properties and device performance.It deserves to be mentioned that the mobility of this WS 2 channel can reach about 184 cm 2 V −1 s −1 at 110 K, outperforming many current transistors based on ultrathin TMD, or nanothick silicon and germanium [40].
Interestingly, an alternative PtBi 2 -contacted transistor, utilizing a thinner WS 2 nanoflake, exhibited relatively commendable transport performance, as shown in figure 3(e).Notably, it presented a significantly reduced Schottky barrier height of approximately 30 meV, as depicted in figure 3(f).This device achieved a lower mobility of 41.76 cm 2 V −1 s −1 and a 10 7 on/off ratio is achieved, with γ value similar to transistor with thicker WS 2 (figure S3).The variation of mobility with thickness of 2D TMDs materials is influenced by many factors such as quantum confinement effect, surface scattering, electron-electron interaction and phonon scattering [41,42].The relative significance of these factors can vary depending on the specific type of material, its preparation method, and the conditions of its application.A smaller Schottky barrier does not invariably lead to enhanced transport performance in a device.The complex interplay of the aforementioned factors can dictate the overall behavior of the transistor.However, these results do provide valuable guidance for future investigations, particularly in measuring the contact potential of materials with varying thicknesses.Such studies could offer deeper insights into optimizing the design and performance of transistors, especially in contexts where the choice of material thickness and its corresponding electronic properties are critical considerations.
To delve deeper into the Schottky barrier height associated with PtBi 2 -WS 2 contacts, the contact potential difference (CPD) was measured, focusing on the dependence on material thickness.Figures 4(a) and (b) showcase the surface topography and surface potential mapping of a typical PtBi 2 contact with varying layers of WS 2 .Flake of similar thickness to that used in the transistor experiments was transferred onto a WS 2 nanoflake with stepped thickness.Subsequently, an Au metal pad was also transferred to the PtBi 2 top surface.The thickness of PtBi 2 was determined to be approximately 170 nm, while the WS 2 varied from 1.6 to 5.6 nm, representing bilayer, four-layer, and six-layer structures, respectively [43].KPFM mapping image measures the relative CPD between PtBi 2 and WS 2 in the same scanned area.Interestingly, as depicted in the line profile (figure 3(b) inset), alters with the number of WS 2 layers, ranging from 128 mV (6 layers) to just 54 mV (bilayer), indicative of effective electron doping into WS 2 within this limited thickness range [19].This variation in potential is inversely related to the work function, suggesting that the work function difference between PtBi 2 and 6 layers of WS 2 (128 meV) aligns closely with the Schottky barrier calculated from the Arrhenius plots (150 meV) of fabricated multilayer WS 2 -based transistors.Consequently, the work function of a 170 nm thick PtBi 2 nanoflake is estimated to be 4.53 eV, with the multilayer WS 2 holding the conduction band position at around 4.4 eV.Further investigations on whether the work function of PtBi 2 varies with its thickness were conducted using ultraviolet photoelectron spectroscopy (UPS).The UPS measurements were performed on bulk crystal films of PtBi 2 with thicknesses exceeding 400 nm.Given the absence of a bandgap in semimetals, a lower work function of 3.95 eV was obtained from the Fermi level line in the UPS spectra (figure 4(c)).This value is even lower than the electron affinity of multilayer WS 2 , implying a negligible barrier height when thick PtBi 2 contacts atomic-thin-layer WS 2 .On one hand, this negligible or even negative barrier height is generally seen as indicative of excellent ohmic contact, promising enhanced transistor performance [44,45].On the other hand, the mobility of WS 2 transistors significantly decreases with reduced thickness.Therefore, by fine-tuning the thickness of both PtBi 2 and WS 2 nanoflakes, there exists potential to further improve the performance of these semimetal-contact transistors.It is important to note that the physical properties of bulk and surface states are usually discussed separately.While carrier dynamics are predominantly governed by the bulk bands of the crystal, the bands near the contact cross-section of PtBi 2 are substantially influenced by the contact substrate or WS 2 , diminishing the effect of the bulk state [46,47].This underscores why thick PtBi 2 flakes remain valuable for both practical applications and research in device technology.
Figure 4(d) shows the expected line-up of the metal Fermi level with the electronic bands of TMDs if only the difference between the electron affinity of TMDs and the work function of the corresponding metal is considered [21,27,28].Comparing Au metal electrodes, PtBi 2 holds a much smaller work function which may result in a smaller Schottky barrier height when in contact with TMDs semiconductors.Electron injection is expected for PtBi 2 to WS 2 and MoS 2 , given that the Fermi level of PtBi 2 is closer to the conduction bands of these TMDs.However, for WSe 2 , PtBi 2 is anticipated to facilitate access to the valence band, leading to hole injection.While this is a projected band diagram, the universal applicability of PtBi 2 contacts to different TMD semiconductors and the accurate electrical properties of PtBi 2 contact transistors still require further empirical testing and characterization.
To substantiate the hypotheses derived from the electronic band diagrams and to demonstrate the universality of In practical industrial production applications, transistor performance is usually improved by precisely controlling the channel length.Therefore, leveraging the tendency of the trigonal crystal system of PtBi 2 to form triangular or trapezoidal shapes during exfoliation, two PtBi 2 flakes with vertically opposite angles were transferred onto a thin-layer WS 2 nanoflake.As illustrated in the SEM photograph inset of figure 5(b), the channel width in this case, defined as the staggered distance of the vertices of a triangle, is just 520 nm.Compared to the transport performance of the thin-layer WS 2based transistor discussed in figure 2, the short-channel device demonstrated still linear output (figure S5) and a tenfold increase in on-state current about 41.6 µA µm −1 .Considering the potential for further reduction in channel length and the substitution of a high-k dielectric layer for a silica layer back gate, the device performance can be further improved.A notable difference in this short-channel transistor is the significantly higher positive threshold voltage (∼19.8V) diverging from the other PtBi 2 -contact transistors, by performing a linear fit to the transfer curve and calculating the ratio of the slope to the intercept.In shorter channel devices, short-channel effects become more pronounced.These include reduced gate control and increased influence of charge at the source and drain ends.This can lead to a shift in the threshold voltage, often requiring a higher voltage to turn the device on.Changes in the material properties of the channel or contact materials in the short-channel transistor compared to the standard device might also contribute.For instance, variations in the work function of the contact material or the intrinsic properties of the semiconductor layer could affect the threshold voltage.Additionally, the altered geometry in the short-channel transistor, particularly the length-to-width ratio of the channel, can influence the electric field distribution in the device, potentially leading to a higher threshold voltage.Further adjustment of the threshold voltage, such as achieving control at a small positive value, could open up possibilities for the practical application of these materials in logic circuits, a direction for future research.
Ultimately, a statistics study was conducted to analyze the dependency of WS 2 thickness on transistor performance, as shown in figure 5(c).Over ten devices were tested, with the highest mobility achieved in a PtBi 2 -contacted multilayer WS 2 transistor at approximately 93.6 cm 2 V −1 s −1.From two layers to eight layers of WS 2 , the average transistor mobility gradually increased from about 21 cm 2 V −1 s −1 to about 85 cm 2 V −1 s −1 .These results align with the observed trends in TMD material mobility variations with thickness.Although the contact barrier tends to decrease as the TMD nanoflakes become thinner, its impact on the overall transistor mobility, as one of several contributing factors, warrants further exploration and analysis.Besides to these valuable research findings, it is important to note that a key limitation in using PtBi 2 single crystals is achieving uniformly shaped and consistently thick flakes through mechanical exfoliation, which is vital for device performance and reproducibility.Additionally, scaling up PtBi 2 production while maintaining its properties poses a significant challenge.Our future research will focus on refining exfoliation methods for PtBi 2 to ensure flake uniformity and exploring scalable synthesis techniques, like advanced chemical vapor deposition, to facilitate its transition from laboratory research to industrial application.Addressing these challenges is crucial for establishing PtBi 2 as a key material in electronic device technology.

Conclusion
This study marks the inaugural demonstration of the novel triply degenerate semimetal PtBi 2 in electronics.Exfoliated PtBi 2 flakes, employed as the interlayer contact with Au electrodes and WS 2 , were integrated via a vdW transfer process to fabricate TMD-based transistors.A key distinction of PtBi 2 , in contrast to previous elemental semimetals and Dirac semimetal ZrTe 2, is its high environmental stability.This characteristic provides a simplified FET device fabrication process and long-term stable working.It notably reduces the Schottky barrier between metal electrodes and TMD semiconductors, while simultaneously ensuring excellent contact by circumventing the Fermi pinning effect typically encountered during metal deposition.A switching ratio above 10 6 and a high average mobility of 85 cm 2 V −1 s −1 are achieved.Importantly, the study reveals that the CPD between PtBi 2 and TMDs semiconductors is influenced by the thickness of both the PtBi 2 and the TMDs layers.This finding provides avenues for further enhancing the performance of PtBi 2 -contact transistors.This research contributes significantly to the selection of suitable semimetals with adjustable work functions for addressing metal-semiconductor contact challenges.Most crucially, it bridges the gap between the physical theory of the novel triply degenerate semimetal PtBi 2 and its practical application, paving the way for its integration into electronic device technology.This pioneering work not only underscores the potential of PtBi 2 in electronics but also sets a precedent for the exploration and application of other novel materials in the realm of semiconductor technology.

Future perspectives
The groundbreaking use of PtBi 2 , a triply degenerate semimetal, as a vdW contact interlayer in 2D transistors, paves the way for significant advancements in electronic device technology.This research, marking the first application of PtBi 2 in such devices, highlights its potential to enhance performance due to its unique properties like low resistivity and environmental stability.Future research is poised to explore diverse PtBi 2 -based device architectures, focusing on optimizing the interplay between device miniaturization and enhanced performance.Given its promising electronic properties, application of PtBi 2 could extend beyond traditional transistors to optoelectronic and spintronic devices.This expansion could exploit high carrier mobility and air stability of PtBi 2 to create more efficient and durable components.Scalability of PtBi 2based transistor for commercial production is another critical area for future investigation.Developing methods for largescale synthesis and integration of PtBi 2 is essential to transition from laboratory research to industrial application.

Figure 1 .
Figure 1.Characterization of semimetal PtBi 2 single crystal and WS 2 contact.(a) Temperature-dependent resistivity of PtBi 2 crystal from 2 to 300 K with a residual resistivity ratio (RRR = R 300 K /R 2 K ) of 145.Inset fit the low-temperature resistivity (<50 K) and residual resistivity ρ 0 ≈ 4.3 µΩ cm.The resistance is measured by the four-wire method.(b) XRD spectra of PtBi 2 crystal with (0 0 l) plane.(c) Raman spectra of PtBi 2 , WS 2 and heterojunction of PtBi 2 -WS 2 contact, respectively.The peaks from WS 2 and PtBi 2 have no obvious shift after contact.Inset: Optical image of PtBi 2 /WS 2 heterojunction.(d) Linear volt-ampere characteristics (V-I) of pristine PtBi 2 flake and retest after 5 months.The resistance is measured by the two-wire method.The resistance (e) and Raman spectrum (f) of four PtBi 2 flakes were measured before and after 5 months.

Figure 2 .
Figure 2. Transport characterization of PtBi 2 contacted WS 2 transistor, (a) Schematic cross-section diagram of the transistor with the lattice structure of both WS 2 and PtBi 2 layer.(b) Optical image of all-physic-transfer PtBi 2 contacted WS 2 transistors.The channel length and width measure approximately 17 and 21 µm, respectively.The thickness of WS 2 and PtBi 2 is about 7 and 170 nm, respectively.(c) Output and (d) transfer curve of transistor under room temperature.The on-off ratio (>10 6 ) is determined from a semi-logarithmic plot of transfer while the mobility of 72.2 cm 2 V −1 s −1 is obtained.

Figure 3 .
Figure 3. (a) Temperature-dependent transport characteristics of PtBi 2 contacted WS 2 transistors.(a) Temperature-dependent transfer curve from 300 to 110 K. V ds = 1 V.(b) Corresponding Arrhenius curve extracted from the temperature-dependent transfer curve with Vgs from −20 to 20 V. (c) Effective energy barrier for different back gate voltage of the transistor.The Schottky barrier height (Φ SB ) is extracted from the dominant change from the thermionic emission current to the thermally assisted tunnelling current.The transition point in this case is about 150 meV.(d) Field-effect mobility as the function of temperature.The red dash lines fit the model µ = T −γ in the 110 to 300 K temperature range.(e) The transfer characterization of PtBi 2 contact transistor with the thickness of WS 2 reduces to about 2 nm.The on/off ratio is above 10 7 with a smaller mobility of 41.8 cm 2 V −1 s −1 .(f) Schottky barrier height extraction from the temperature-dependent transfer characterization as well as the Arrhenius curve.Smaller Φ SB of 30 meV is obtained when PtBi 2 contact a thin WS 2 nanoflake.

Figure 4 .
Figure 4. Characterization of the PtBi 2 work function and contact potential difference to TMDs.(a) Atomic force microscopy characterization of PtBi 2 contact WS 2 with different layers.PtBi 2 flake with a thickness of 170 nm is similar to the flakes used in transistors.The white dash line demarcates the WS 2 with different thicknesses.The thickness of WS 2 varies from 1.6 to 5.6 nm representing bilayer to 6 layers.(b) KPFM characterization of WS 2 with different layers contact with PtBi 2 .The work function of PtBi 2 is determined as 4.53 eV.CPD decreases with the thickness of WS 2 reducing.(c) UPS spectra of PtBi 2 bulk crystal.The Fermi level is determined at 3.95 eV.(d) The expected (not correct) line-up of electronic bands of multilayer TMDs flakes if only the difference of the electron affinity of TMDs and the work function of PtBi 2 is considered.

Figure 5 .
Figure 5. Universality and repeatability characterization of PtBi 2 contact TMDs transistors.(a) Transfer characterization and on-state current calculation of PtBi 2 contact WS 2 , MoS 2 or WSe 2 under room temperature.The abscissa axis is expressed by Vgs-V T , and the vertical axis is expressed by the transfer current dividing channel width.Inset: The linear output of PtBi 2 contact TMDs transistors.(b) Transfer characterization as well as on-state current determination of short channel transistor.The threshold voltage is 19.8 V. Inset: optical image of short channel transistor with a channel width of about 520 nm.(c) Statistical analysis of the relationship between thickness of WS 2 and mobility of PtBi 2 contact transistors.