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Shubhrasish Mukherjee, Gaurab Samanta, Md Nur Hasan, Shubhadip Moulick, Ruta Kulkarni, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), [Arumugum Thamizhavel](https://orcid.org/0000-0003-1679-4370), Debjani Karmakar, [Atindra Nath Pal](https://orcid.org/0000-0001-9584-2283)

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[Achieving nearly barrier free transport in high mobility ReS2 phototransistors with van der Waals contacts](https://mdr.nims.go.jp/datasets/d2d17e6d-5383-4483-abe4-697d1be2d11c)

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Achieving nearly barrier free transport in high mobility ReS2 phototransistors with van der Waals contactsnpj | 2D materials and applications ArticlePublished in partnership with FCT NOVA with the support of E-MRShttps://doi.org/10.1038/s41699-024-00507-3Achieving nearly barrier free transport inhigh mobility ReS2 phototransistors withvan der Waals contactsCheck for updatesShubhrasish Mukherjee1,2 , Gaurab Samanta1, Md Nur Hasan3, Shubhadip Moulick2, Ruta Kulkarni4,Kenji Watanabe 5, Takashi Taniguchi 6, Arumugum Thamizhavel 4, Debjani Karmakar3,7,8 &Atindra Nath Pal 1,2Focusing on Rhenium disulfide (ReS2), a group VII transition metal di-chalcogenides (TMDC), being apromising contender system for future nanoelectronics and optoelectronics, here, we present aninnovative pathway to experimentally achieve an almost barrier-free contact for the ReS2 field effecttransistors (FETs) by using few layered graphene as contact electrodes, further supported bycomparative first-principles analysis. Such barrier-free contacts enable the observation of metal-to-insulator transition with enhanced room temperature carrier mobility up to 25 cm2/Vs, linear Ids-Vdscharacteristic down to 80 K, along with the reduction of 1/f noise by more than two orders ofmagnitude.We further demonstrate a highly responsive gate- tunable phototransistor (R > 106A/W) atan illuminationwavelength of 633 nm. Thiswork demonstrates a straightforward strategy to unlock thefull potential of ReS2 for CMOS compatible future electronic and optoelectronic devices.The fascinating physical, electrical, and optical properties of the two-dimensional (2D) layered materials1,2 like graphene, transition metal di-chalcogenides (TMDCs), etc. have promoted their utilization as promisingcandidates for post-silicon era nanoelectronics and optoelectronics. As perits definition, 2D materials have strong in-plane chemical bonds but weakout-of-plane van der Waals interactions3. This unique feature is bestemployed in the stacked assembly of van der Waals architectures afterignoring their lattice misalignment and thus helps to potentially obtain anunprecedented device functionalities, thus far unavailable in the devicesmade out of the individual components of the assembly4,5. Recent studies onthe structure-property relationship of the widely studied group VII TMDCmaterial6,7, Rhenium disulfide (ReS2), have revealed that the flat valence-band edges of bulk and few-layered ReS2 have resulted in a near-direct bandgap system ranging from 0.8–1.5 eV depending upon the thickness of thelayers8,9. Therefore, few-layered and bulk ReS2 possess the potential for asignificant light absorption and emission10. More importantly, the in-planeanisotropy provides an additional degree of freedom to create anisotropicelectronic11–13 andoptoelectronic devices14 using 2D integrated circuit. ReS2-based field-effect transistors (FETs) and photodetectors have beendemonstrated on several occasions in the prior literature15,16. However, thedevice performance is significantly altered depending upon the highercontact resistance (Rc), lower electron mobility (μ), and lower on/off ratio.The primary reasons behind this underperformance are either related to ahigh Schottky barrier related to a mismatch between the metal work func-tion and semiconductor electron affinity, or it may be related to the Fermilevel pinning effect, induced by metal induced gap states (MIGS)17. Thesemiconductor industry has been facing a growing number of critical issuesrelated to electrical contact resistance at the metal-semiconductorinterfaces18. These issues are impeding the ultimate scaling and perfor-mance of electronic devices. Achieving low contact resistance by loweringthe Schottky barrier height (SBH) between semiconductors and contacts,which is intimately tied with the carriermobility, is a crucial prerequisite forthe realization of the next generation 2D electronic and optoelectronicdevices. In recent years, significant efforts have beenmade to address issuesrelated to metal-semiconductor contacts19,20. Some notable approachesinclude utilizing contacts with various low work function metals21, phaseengineering22, introducing high-k dielectrics23, fabricating edge contacts24,and employing van der Waals contacts25–27. However, most of these1Technical Research Centre, S. N. Bose National Centre for Basic Sciences, Kolkata, India. 2Department of Condensed Matter and Material Physics, S. N. BoseNational Centre for Basic Sciences, Kolkata, India. 3Department of Physics and Astronomy, Uppsala University, Uppsala, Sweden. 4Department of CondensedMatter Physics and Material Science, Tata Institute of Fundamental Research, Mumbai, India. 5Research Center for Electronic and Optical Materials, NationalInstitute for Materials Science, Tsukuba, Japan. 6Research Center for Materials Nano architectonics, National Institute for Materials Science, Tsukuba, Japan.7Technical Physics Division, Bhabha Atomic Research Centre, Mumbai, India. 8Homi Bhabha National Institute, Trombay, Mumbai, India.e-mail: shubhra@bose.res.in; debjan@barc.gov.in; atin@bose.res.innpj 2D Materials and Applications |            (2024) 8:71 11234567890():,;1234567890():,;http://crossmark.crossref.org/dialog/?doi=10.1038/s41699-024-00507-3&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41699-024-00507-3&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41699-024-00507-3&domain=pdfhttp://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0003-3701-8119http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0002-1467-3105http://orcid.org/0000-0003-1679-4370http://orcid.org/0000-0003-1679-4370http://orcid.org/0000-0003-1679-4370http://orcid.org/0000-0003-1679-4370http://orcid.org/0000-0003-1679-4370http://orcid.org/0000-0001-9584-2283http://orcid.org/0000-0001-9584-2283http://orcid.org/0000-0001-9584-2283http://orcid.org/0000-0001-9584-2283http://orcid.org/0000-0001-9584-2283mailto:shubhra@bose.res.inmailto:debjan@barc.gov.inmailto:atin@bose.res.inwww.nature.com/npj2dmaterialstechniques to enhance contact characteristics have primarily been exploredfor group VI transition metal dichalcogenide (TMDC) transistors28–30.There are considerably fewer reports on achieving smooth interfaces withlower SBH for group VII TMDC layers.In this report, we propose an approach to achieve a barrier-free contactfor ReS2 FETs, after employing few layered graphene as a tunable contact.The van derWaals contact further assures aminimal interfacial strain at theReS2-graphene interface. At cryogenic temperatures down to 80 K, wedemonstrate that graphene is capable of forming a transparent contact withReS2, achieving a nearly ohmic nature of conduction. TheReS2 devices (RG)with graphene contact offer a high charge carriermobility of ~25 cm2/Vs ineven room temperature. First-principles investigation of ReS2-grapheneinterface indicates its unique performance over othermetallic contacts afterretaining the band-gap of ReS2 to be devoid of anyMIGS. Furthermore, theachievement of a low resistance and the ohmic nature of the contact allowfor the investigation of the intrinsic properties of ReS2 as a channelmaterial,exhibiting a metal to insulator (MIT) phase transition at low temperatures.Interestingly, compared to the conventionalmetal contacted (Ti/Au andCr/Au)ReS2 devices, RGdevices offer a lower1/f noise (~10−7Hz−1 at 1 Hz) andhigher photoresponse (>106A/W at 633 nm). Our approach of employinggraphene as a barrier-free contact may provide insight into contact engi-neering in conventional semiconductors in general as well as in atomicallythin semiconductors specifically.Results and discussionsElectronic transportFigure 1a represents the schematic illustration and the optical image of thehybrid device where few layered ReS2 is used as a channel material and fewlayered graphene as the electrode. The schematic of the electronic structureand the Raman spectra of few layered ReS2 are represented in Supple-mentary Fig. 1. Few layered hexagonal boron nitride (hBN) is the substrate,which helps to reduce the charge impurities and the scattering from thesubstrate phonons31. Supplementary Figs. 2 and 3 show atomic forcemicroscopy (AFM) images of the individual materials, demonstrating theiruniformity throughout the experiments. All these devices are fabricated byusing a pickup and attached-based dry transfer technique (See “Experi-mental Methods” for details). As an initial step, two distinct metal contactslike Ti/Au and Cr/Au are used to examine the electrical characteristics of afew-layered ReS2 FET. Both devices exhibit an n-type behavior, similar topreviously reported11,14 results, with moderate on/off ratios (>104) andcarriermobilities (<3 cm2/Vs). The insertion of few-layered graphene sheetsbetween theReS2 channel and themetal contacts is seen to have a significantimprovement in then-type conduction, resulting in ahigheron/off ratio andfar higher charge carrier mobilities. Figure 1b shows a comparison of thetypical transfer characteristics, displaying the drain-source current (Ids)versus gate-source voltage (Vbg) for Cr/Au (RCr), Ti/Au (RTi), andgraphene-contacted (RG) field-effect transistors (FETs) made of few-layered ReS2. The on/off ratio and the field effect mobilities (μ) of thesedevices are compared in Fig. 1c and d respectively. Remarkably, the on/offratio of RGdevice exceeds 105, which is nearly 3.5 times higher thanRTi and4.5 times higher than RCr devices. Moreover, for RG devices, the measuredvalue of μ reaches 25.17 cm2/Vs, which is one of the highest achieved valuesamong the reported back-gated few-layered ReS2 FETs in roomtemperature12,32–35. These improvements can be attributed to the improvedcharge carrier injection through the graphene contacts, which results indecreased contact resistance due to the lowering of the effective height of theSchottky barrier between the electrodes and ReS2 channel resulting into areduced Fermi level pinning. A noticeable negative shift in the thresholdvoltage (VT) is observed for graphene contacted devices at room tempera-ture (Supplementary Fig. 4 and Table 1 in the Supplementary Material).This suggests enhanced electron doping in ReS2 channel due to the efficientcharge transfer from graphene36. We analyse these results by using a first-principles electronic structure investigation of the interfaces of bilayer ReS2withmultilayers of Cr [001] (RCr) andTi [001] (RTi) surfaces and alsowithgraphene (RG). The method of construction of these interfaces and thecalculation of themutual interfacial strain is described in the SupplementaryMaterial (Note 4). Themutual strain for these three interfaces is 6.3, 6.1, and0.98% respectively, implying a minimal interfacial strain for the RGFig. 1 | Comparison of room temperature characteristics of ReS2 transistors withdifferent contacts. a Schematic of the hybrid device. Optical image of the device(upper left inset). Schematic of vertical hBN-ReS2-graphene interface (upper rightinset). b Comparison of transfer characteristics of the ReS2 FETs with differentcontacts atVds = 1 V.Comparison of (c) on/off ratio and (d)field effectmobilities (μ)of ReS2 FETs with different contacts. Calculated electronic band structure of (e)ReS2-Cr and (f) ReS2-Ti interfaces, theMiddle panel displays a schematic of the banddiagram for metal-ReS2 interfaces. g Schematic band diagram and calculated bandstructure of ReS2-graphene interface.https://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 2www.nature.com/npj2dmaterialsinterface. Figure 1e and f depict the electronic band structures of theRCrandRTi interfaces respectively with an associated schematic band-alignmentdiagram. The comparison of the Fermi-energies of these three interfacialsystems with pristine bilayer ReS2 reveals that all the three interfacial sys-tems are n-type doped with the extent of doping being highest for RGsystem, resembling the experimental results. For the metallic contacts, theformationof theCr-S andTi-S covalent bonding at the interfaces, the relatedcharge transfer and structural relaxations, leads to the occurrences ofMIGS37within the gapsofReS2, as indicatedby thehighlighted energy-rangein the respective band dispersions with the Cr (Cyan) and Ti (magenta)atom-projected bands (Supplementary Fig. 6). The RG interface, on theother hand, results into a far cleaner interface, consisting of severaladvantages over the other two contacts28,36. First, this interface is devoid ofmuch structural distortions due to minimal interfacial strain. Second, thevan der Waals nature of graphene keeps the interface free of any chemicalbonding. Third, the band-alignment, charge transfer, and respective com-parison of the Fermi-levels indicate that the Dirac crossing of graphene isenergetically situated deep inside the conduction band of ReS2 due to thelarge p-type doping of the graphene contact, as indicated by the C (blue)atom-projected bands in the Fig. 1g38. The atom-projected band structuresfor ReS2-graphene and ReS2-graphite interfaces, as presented in Supple-mentary Fig. 7 also keep the same trend of placement of Dirac crossingwithin the conduction bandofReS2. This leads to a suppression of theMIGScontributed by the conduction band,where the electrons fromgraphene canbe easily transferred to the conduction band of ReS2. The valence band, ontheother hand, remains completelyfilledand thusdoesnot contribute to theproduction of MIGS. Thus, in RG interface, the gap of ReS2 remainscompletely intact without the presence of any MIGS. As a result, thesemiconductor in contactwith graphenewill be in a degenerate state and theinterface will be nearly free of a Schottky barrier. This theoretical analysisprovides a physical explanation to the experimental outperformance of theRG devices over the other two metallic contacts. We measure severaldevices, and almost similar results have been observed (See Table-1 inSupplementary Material).As a next step, the temperature dependent transfer characteristics of alldevices are studied in order to quantify their Schottky barrier heights (SBH),as presented in Fig. 2a–c (Supplementary Fig. 8). The drain current (Ids) in atwo-terminal RG FET increases as the temperature decreases from roomtemperature (300 K) to 87 K due to the enhanced electronmobility of ReS2,indicating ametal insulator transition (MIT) atVbg > 20V. In contrast, Ids inboth RTi and RCr FETs decreases dramatically with temperature due to adecrease in thermionic emission current at contact. The cooling-inducedincrease in the on-current (metallic type) demonstrates that the device isdominated by the intrinsic electrical transport property of the ReS2 channelduring the cooling process rather than the contact resistances of the ReS2-graphene interfaces39. The temperature dependent field effectmobility (μ) isrepresented in Supplementary Fig. 9 which can be calculated by usingμ ¼ LWCtotal1VdsdIdsdVbgð1ÞFig. 2 | Low temperature transport and Schottky barrier height. Temperaturedependent transfer characteristics (Ids-Vbg) of (a) graphene (RG), (b) Ti/Au (RTi)and (c) Cr/Au (RCr) contacted ReS2 devices at Vds = 250 mV. (d–f) Arrhenius plotsand (g–i) the calculated Schottky barrier height (SBH) of the devices at different gatevoltages with Vds = 250 mV. The SBHs are extracted under flat band gate voltagecondition. Negligible SBH are found in RG FET whereas, a SBH of ~83 meV for RTiand 96 meV for RCr are extracted respectively. jCalculated contact resistances of thedifferent devices.https://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 3www.nature.com/npj2dmaterialswhere L and W are the channel length and width respectively, Ctotal is thetotal capacitance which is basically a parallel combination of 285 nm SiO2and ~14 nm of hBN.Increasing the temperature from 87 K to 300 K, μ decreases from91 cm2/Vs to 25.17 cm2/Vs which can be explained by the dominantelectron-phonon scattering40. The temperature dependent mobility can befitted by the generic equation,μ � T�ν ð2ÞWhere ν is the exponent that depends on the dominant phononscattering mechanism and is determined to be 1.2, which is consistent withpreviously reported results for the other materials40,41. Then, the SBH arecalculated by using the modified Richardson equation,Ids ¼ A *T32 exp � ϕBKBT� �ð3Þwhere, A* is the Richardson constant for 2D system, T is the absolutetemperature, KB is the Boltzmann constant, ΦB is the Schottky barrierheight42. Using this Eq. (3), the Arrhenius plot of ln(Ids/T3/2) vs 1000/T atdifferent Vbg are plotted in Fig. 2d–f for different contacts of ReS2 FETs. RGdevices show opposite trends compared to RTi and RCr devices. Thepositive slope in RG device indicates the deviation from the thermionicemission model and a negligible Schottky barrier height at ReS2-grapheneinterface. The effective Schottky barrier height (SBH) is then calculated fordifferent contacted devices under the flat band gate voltage (Vbg) condition(Fig. 2g–i). The extracted SBH becomes ~83meV for Ti/Au and ~96meVfor Cr/Au devices respectively, whereas almost zero SBH is observed ingraphene contacted devices. Then, the contact resistances of the twoterminal devices are then calculated by Y function method43,44 (See sup-plementary note 8 for details). For RG FETs, the contact resistances arefound to be approximately 14.5 kΩ-μm, nearly three orders of magnitudelower compared to RTi and RCr FETs (at Vbg-VT ~ 30 V), as illustrated inFig. 2j, marking one of the most significant advancements in reducingcontact resistance for ReS2 FETs45–47.Finally, the Ids-Vds characteristics of the devices are investigated atvarious temperatures. Figure 3a and b depict the typical room temperatureand low temperature (84 K) Ids-Vds characteristics of RG FETs at variousVbg. At all temperatures (84–300K), the RG FET exhibits linear outputcharacteristics, indicating Ohmic conduction48,49. Supplementary Fig. 11depicts the Ids-Vds characteristics of RTi and RCr FETs at room and lowtemperatures. The nonlinear output characteristics imply the presence of afinite Schottky barrier height at the ReS2-Ti andReS2-Cr interfaces, which istypical for such devices21,50. To understand more clearly, a simple mathe-matical parameter called nonlinearity (N) factor can be introduced as28N ¼ ðd2Ids=dV2dsÞ=2ðdIds=dVdsÞ ð4ÞFor a linear Ids-Vds characteristics,N ~ 0, which suggests no significantSBH present. Furthermore, large N indicates increased nonlinearity, whichis associated with a higher SBH. A higher N indicates more nonlinearity,which correlates with a higher Schottky barrier. Figure 3c depicts non-linearity (N) as a function of temperature in all three types of devices (Vds =300mV). It is observed thatN increaseswithdecreasing temperature forRTiand RCr, whereasN remains close to zero at all temperatures for RG. Thus,the nearly barrier-free contacts in the ReS2-graphene interface are onceagain supported by N ~ 0 in RG FET. As previously discussed, the reducedFermi level pinning near the band gap of the ReS2 channel accounts for theexceptional electrical contact and barrier-free transport. Additionally, these2D integrations facilitated by the dry transfer method circumvent thenecessity for direct metal deposition atop the ReS2 contact region, in con-trast to the traditional ReS2-metal interfaces. The graphene and ReS2interface are atomically sharp and ultraclean thanks to this nondamagingvan der Waals bonding technique, which also reduces interface chargetrapping states and prevents Fermi level pinning, which is the dominant intraditional metal-ReS2 contacts. The SBH has an explicit relationship withthe vertical tunneling barrier across the interface. To validate the nature ofthe SBH in these three interfaces as obtained from the transport measure-ments, we have calculated the effective electrostatic potential (Supplemen-tary Fig. 12) of the three interfacial systems RCr, RTi, and RG. The upperpanels of the Fig. 3d–f represent the c-axis-stacked interfaces RCr, RTi, andRG respectively. The lower panels of the correspondingfigures represent thetheoretically calculated one-dimensional projection of the effective elec-trostatic potential with respect to the stacking axis, which, for the presentcase is the c-axis. As will be evident from the Fig. 3d and e, the electrostaticpotential difference (Φ) between ReS2 and the metal (Cr/Ti) is positive,implying that the charge carriers from the ReS2 channel needs to cross abarrier to get transferred to themetal. On the contrary, Fig. 3f indicates thatthe same potential difference (Φ) between ReS2 and graphene is negative.Therefore, the charge-carriers from ReS2 do not have to encounter anybarrier to get easily transferred to the vertical graphene layer. Figure 3g–ipresent the atom-projected (APDOS) and total density of states (DOS) forRCr, RTi, andRG interfaces respectively, theoretically extracted byusing thetetrahedron method. Similar to the band-dispersions, both RCr and RTidisplay localized Cr-d and Ti-d levels, which are highly hybridized with theSulfur-p states within the gap of ReS2, resulting into the presence of MIGS.The RG system, however, evinces the intact band gap of ReS2 and is devoidof any localized graphene-induced levels.Optoelectronic transportTo examine the impact of improved electronic behavior via contact engi-neering on optoelectronic performance, we systematically investigated thephotodetection capabilities of ReS2 FET devices for the three types of con-tactsmentionedbefore. All the optoelectronic experiments are performed atroom temperature under 633 nm light irradiation. Figure 4a depicts thetransfer characteristics of a graphene-contacted ReS2 FET (RG) under dif-ferent effective illumination powers (λ = 633 nm) at Vds = 1 V (for results ofRTi and RCr devices, see Supplementary Fig. 13). The mechanism ofphotocurrent generation is represented in Fig. S14a (see SupplementaryMaterial note 12). A monotonic increase of the drain current (Ids) isobserved with the illumination intensities used throughout the window ofgate voltage sweep, as indicated by the vertical shift of the transfer char-acteristics. Concomitantly, as light power increases, the threshold voltage(VT) shifts towards the negative direction (Supplementary Fig. 14b). Thevertical shift in the transfer characterization is due to the generation ofelectron-hole pairs by incident photons, also knownas thephotoconductionor photoconductive effect51, while the horizontal shift in the transfer char-acteristics represents the photogating effect51. The photocurrent (Iph) wasobserved to exhibit a straightforward power-law relationship with incidentpower (PLED), expressed as Iph ∝ PLEDα, where α falls within the range of0.3–0.4 (Supplementary Fig. 14c). The non-unity exponent arises from theintricate interplay of electron-hole generation, trapping, and recombinationprocesseswithin the semiconductor52. Themost importantfigure ofmerit ofsuch photodetector devices is photoresponsivity (R)53,54, defined as the ratioof photocurrent generated (Iph) to effective illumination intensity (PLED),i.e.,R ¼ IphPLEDð5ÞAs depicted in Fig. 4b, R shows a steady rise with rising gate voltageacross each light intensity level. With gate voltages adjusted from−40 V to30 V while maintaining λ = 633 nm, PLED ~ 0.9 μW/cm2, and Vds = 1V, Rincreases nearly three orders of magnitude, surging from 4.93 × 103A/W to1.21 × 106A/W. The gate tunable R for RTi and RCr are represented inSupplementary Fig. 13c and d, respectively, displaying qualitatively similarbehavior. Figure 4c represents the comparison of R for three devices atλ = 633 nm, Vds = 1V, Vbg-VT ~ 10 V. Across all devices, the anticipatedinverse relationship between R and PLED is evident55. Notably, RG exhibitshttps://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 4www.nature.com/npj2dmaterialssuperior photoresponse compared to the other devices, albeit followingsimilar trends.Under identical experimental conditions, RG achieves a peakR value of 1.1 × 106A/W, while RTi and RCr attain maximum R values of7.7 × 104A/W and 8.9 × 103A/W, respectively. These findings underscorethat the graphene contacted devices fabricated in this study rank among thetop-performing photodetector devices utilizing ReS2 layers reported todate56–58 (See Table-2 in Supplementary Material). Figure 4d–f depict thetemporal photoresponse for the fabricated devices during one cycle of lightmodulation under the same experimental conditions of Vds = 1 V,Vbg = 0 V, λ = 633 nm, and PLED ~ 100 μW/cm2. The time response for thephotocurrent decay is dominated by the two components which are mostlikely associated with the multiplicity of electron traps in the ReS2 channelfrom different surface states59. For RG, RTi, and RCr, the faster decaycomponents are 220ms, 440ms, and 480ms, respectively, while the slowerdecay components are 3.59 s, 5.44 s, and 5.69 s (Supplementary Fig. 15). Inaddition to R, the two most crucial parameters for assessing the detectionlimit of photodetectors are the noise equivalent power (NEP) and the spe-cific detectivity (D*). NEP is mathematically defined as53,NEP ¼ffiffiffiffiSIpRð6Þwhere, SI represents the total noise spectral density encompassing 1/f noise,thermal noise, and shot noise. Similarly, the specific detectivity of a devicecan be defined as54,D� ¼ffiffiffiffiApNEPð7Þwhere, A is the device area.Fig. 3 | Ids-Vds characteristics and mechanism of Ohmic like contacts in ReS2-graphene Interfaces. Ids-Vds characteristics of RG FET at (a) room temperature(290 K), (b) 84 K. c Nonlinearity (N) as a function of temperature for the devices.Interfacial stacking and the one-dimensional projection of the effective electrostaticpotential for all three interfacial systems, (d) ReS2-Cr, (e) ReS2-Ti, and (f) ReS2-graphene respectively. Total density of states (DOS) and atom-projected DOS of (g)ReS2-Cr, (h) ReS2-Ti and (i) ReS2-graphene respectively using DFT with GGA-PBEfunctional in VASP.https://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 5www.nature.com/npj2dmaterialsThe NEP and D* are assessed and compared by focusing on the pre-dominant 1/f noise arising primarily due to the slowly varying intrinsicdisorder or interfacial trapping-detrapping processes60–62. Given the sus-ceptibility of these 2D devices to 1/f noise (see SupplementaryMaterial note14 for details), which constrains their detectivity, evaluating the level of 1/fnoise is pivotal. A comparison of normalized current noise power spectraldensity (SI=I2) (at 1 Hz) for the different devices are represented in Fig. 4g.Remarkably, compared to RTi and RCr devices, the noise magnitude of RGreduces by almost two orders of magnitude, being 8.3 × 10−8 Hz−1,7.83 × 10−6 Hz−1 and 8.70 × 10−6 Hz−1 at Vds = 1 V and Vbg-VT ~ 10 V, forRG, RTi and RCr devices, respectively. This can further be visualized fromthe time domain current fluctuation data (δIds=Ids) exhibiting much lowerfluctuation in RG device compared to the other two (SupplementaryFig. 16a). To calculate the NEP and D* at all gate voltages, the gatedependenceof noisewasmeasuredshowing similar 1/f typeof behaviorwithmonotonic decrease of noise with increasing gate voltage (see Supplemen-tary Fig. 16 for details)63. The observed noise reduction can most likely beexplained by the reduced contact resistance and improved channelmobilityin RG64,65. Considering the influence of 1/f noise, the calculatedNEP andD*are illustrated in Supplementary Fig. 17. Results indicate that the NEP andD* for RG are 9 × 10−17W/Hz0.5 and 3.2 × 1012 Jones, respectively. Con-versely, for RTi and RCr, the NEP values become 5.8 × 10−15W/Hz0.5 and1.4 × 10−14W/Hz0.5, with corresponding D* values of 6.5 × 1010 Jones and3.9 × 1010 Jones, respectively. Hence, RG clearly outperforms the otherdevices in terms of its exceptional weak light detection capabilities.In summary, we have shown that the use of graphene electrodes withatomically clean interfaces can lead to a nearly barrier-free contact in ReS2with almost ohmic nature of the conduction. This device configurationdrastically lowers the contact resistance, resulting in an enhanced chargecarrier mobility (~25 cm2/Vs) and an outstanding on/off current ratioexceeding 105 at room temperature. Additionally, the improved contactfacilitates access to the ReS2 channel, enabling the observation of a metal-insulator transition driven by electron-phonon coupling even in a two-terminal configuration. First-principles analysis of these three interfacialsystemsRCr,RTi, andRGreveals the absence ofMIGS in case ofRGdevices,which is in line with the experimental observation.We further demonstratethat superior optoelectronic performances of the graphene-contacteddevices exhibitinghigherphotoresponsivity >106A/W,beingmore than twoorders of magnitude higher compared to the Ti or Cr contacted devices.More importantly, improved electronic behavior leads to a substantialreduction of low-frequency electrical noise by more than two orders ofmagnitude, resulting in significantly lower noise equivalent power of9 × 10−17W/Hz0.5, and a higher specific detectivity of 3.2 × 1012 Jones at633 nm. We believe, our approach of employing graphene as a tunablecontact provides a new pathway toward contact engineering for 2D semi-conductors and can enable newopportunities for future electronics and low-temperature quantum transport in 2D materials.Experimental methodsSynthesis of ReS2 single crystalA two-step process has been adopted for the growth of ReS2 Single crystals.High purity Re and sulfur powders were thoroughly mixed in an agatemortar and cold pressed into a pellet. This pellet was then loaded into athick-walled quartz ampoule and sealed in a vacuum of ~10-6Torr. Thequartz ampoulewas thenheated in a box type furnace by slowly ramping thefurnace to 1150 °C at the rate of 15 °C/hr. and held at this temperature forabout 72 h. before cooling it to room temperature. This pre-reactedmaterialwas then subsequently used for the chemical vapor transport. About 1 g ofthe pre-reacted ReS2 powder along with I2 (150mg) was taken in a 25 cmlong quartz tube which was sealed in a vacuum of about 10−6Torr. TheFig. 4 | Optoelectronic characteristics of ReS2 FETs. a Transfer characteristics (Ids-Vbg) of graphene contacted ReS2 device (RG) under different illumination power(PLED) of λ = 633 nm, Vds = 1 V. bGate dependent photoresponsivity (R) of RG FETas a function of PLED. cComparison of photoresponsivity of three different contactedReS2 FETs at same experimental conditions of λ = 633 nm,Vds = 1 V,Vbg- VT ~ 10 V.Temporal photoresponse of (d) graphene contacted, (e) Ti/Au contacted, (f) Cr/Aucontacted ReS2 FETs at λ = 633 nm, PLED ~ 100 μW/cm2, Vds = 1 V, Vbg = 0 V.g Comparison of 1/f noise characteristics of the fabricated devices in same experi-mental conditions, Vds = 1 V, Vbg-VT ~ 10 V.https://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 6www.nature.com/npj2dmaterialsquartz tubewas placed in a two-zone furnace. The hot zone wasmaintainedat 1000 °C and the cold zonewasmaintained at 960 °C and this gradientwasmaintained for about 8 days to enable the vapor transport and recrystalli-zation of the material. Several nucleation happened and tiny flakes of ReS2single crystals crystallized at the cold end of the crucible.Device fabricationEach of the 2Dmaterials, including the graphene (suppliedby SPI Supplies),layered hBN (supplied fromNIMS, Japan), and few layered ReS2 flakes, aremechanically exfoliated from their bulk crystals by using scotch tape on Si/SiO2 substrate. The typical thickness of the ReS2 flakes is ~5 nm in all thefabricated devices. To ensure uniformity in the experiments, graphene, andhBN flakes are chosen to be approximately 4 nm and 14 nm thick in alldevices respectively (Figs. S2 and S3). Then, the different hybrid stacks(hBN-ReS2/ hBN-ReS2-graphene) are fabricated by standard pick up andattach based dry transfermethod66. The few-layered hBN is awide band gap(approximately equal to 6 eV) semiconductor, which acts as the bottominsulating surface with fewer defects than SiO2 and also provides anatomically flat platform for ReS2 channels. The few layered graphene is puton top of ReS2 which acts as the contact electrodes. The electrical contactsare fabricated by laser writer lithography (LW405D-MICROTECH) tech-nique followed by metallization with e-beam evaporation of Ti/Cr-Au(5–50 nm) with a well-controlled deposition rate (>0.5 Å/s) at 10−6mbar.The channel length of all the devices are in the range of 5–7 μm. All thedevices are annealed at 300 °C in thepresenceofAr-H2gas for 3 h inorder toget better electrical contacts.Materials and devices characterizationsAll the exfoliated 2D materials are characterized by Raman spectroscopy(LabRam HR Evolution; HORIBA, France with 532 nm laser) and atomicforcemicroscopy (AFM) (di INNOVA). Electrical transportmeasurementsare performed at high vacuum (>10−5mbar) inside a homemade cryogenicinsert. A Lake Shore temperature controller (LS336) is used to control andmeasure the temperature. Optoelectronic experiments are carried in ahomemade electronic setup having an optical window. An MFLI lock-inamplifier (Zurich Instruments), a Keithley 2450 source-meter along with acurrent to voltage preamplifier (Femto DLPCA-200) are used in all of theexperiments with an AC two-probe configuration at a carrier frequency of83.67Hz. For the photocurrent measurements we used well-calibrated andcollimated 633 nmLED (Thorlab) sourced by aDC2200 power supply witha spot size of ∼3mm. The LED powers are calibrated by using a Flame-Ocean Optics spectrometer with an integrating sphere setup.Computational methodsWe have investigated its electronic properties from spin-polarized firstprinciples density functional theory. The calculations are performed usingprojector augmented wave (PAW) potentials. The valence orbitals of Rhe-nium(Re) include5p, 6s, and5d, while thoseof Sulfur (S) comprise 3s and3porbitals. The exchange-correlation interactions are treated after using thegeneralized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional, implemented inVASP67,68. Plane-wave cut-off energy is set as 500 eV and Monkhorst-Packmeshes 5 × 5 × 3 are used for electronic structure calculation. The van derWaals interactions are implemented by using DFT-D3 method ofGrimme69.The calculationof effective electrostatic potential and relatedpropertiesare computed using QuantumATK 15.1 package as described in our earlierwork70. The calculations apply double-zeta polarized basis sets with GGA-PBEexchange correlation at an electron temperature of 300 K.TheDFT-D3method is employed to incorporate the influence of nonlocal dispersiveforces, such as the van der Waals interactions. A refined k-point grid of5 × 5 × 3 was utilized, followed by a broadening of 0.1 eV.The effective electrostatic potential is calculated by using the relationVE rð Þ ¼ � VH rð Þe and the electrostatic difference potential byδVE rð Þ ¼ � δVH rð Þe . The Hartree potential VH rð Þ is calculated from thePoisson equation as, ∇2VH ½n� rð Þ ¼ � e24πε0nðrÞ, where, n rð Þ ¼ δn rð ÞþPNatomsI nIðrÞ, δn rð Þ is known as electron difference density, nIðrÞ is thecompensation charge of atom I and Natoms is the number of atoms.Data availabilityData sets of the current study are available from the corresponding authorsupon reasonable request.Received: 8 July 2024; Accepted: 23 October 2024;References1. Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A.Single-layer MoS2 transistors. Nat. Nanotechnol. 6, 147–150 (2011).2. Novoselov, K. S. et al. A roadmap for graphene.Nature 490, 192–200(2012).3. Wang, Q. H., Kalantar-zadeh, K., Kis, A., Coleman, J. 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Bose National Centre forBasic Sciences, established under the TRC project of Department ofScience and Technology (DST), Govt. of India. A.N.P. acknowledges DSTNano Mission: DST/NM/TUE/QM-10/2019. M.N.H and D.K. would like toacknowledge the BARC ANUPAM supercomputing facility forcomputational resources. K.W. and T.T. acknowledge support from theJSPS KAKENHI (Grant Numbers 21H05233 and 23H02052) and WorldPremier International Research Center Initiative (WPI), MEXT, Japan.Author contributionsS.M. andA.N.P. conceived theproject. S.M.designed theexperiments. S.M.and G.S. fabricated the devices and performed the characterization. S.M.and G.S. performed the detailed transport measurements. S. Mo. helped toset up the low temperature measurements. S.M. performed the completedata analysis. R.K. and A.T. have grown and characterized the ReS2 singlecrystal. K.W. and T.T. have grown and supplied the hBN single crystal.M.N.H. andD.K. provided theoretical support andwrote the theoretical part.S.M. wrote the original manuscript. A.N.P. and D.K. supervised the project,validated the analysis, and reviewed and edited the manuscript. All authorshave read and approved the final version of the manuscript.Competing interestsThe authors declare no competing interests.Additional informationSupplementary information The online version containssupplementary material available athttps://doi.org/10.1038/s41699-024-00507-3.Correspondence and requests for materials should be addressed toShubhrasish Mukherjee, Debjani Karmakar or Atindra Nath Pal.Reprints and permissions information is available athttp://www.nature.com/reprintsPublisher’snoteSpringerNature remainsneutralwith regard to jurisdictionalclaims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution-NonCommercial-NoDerivatives 4.0 International License,which permits any non-commercial use, sharing, distribution andreproduction in any medium or format, as long as you give appropriatecredit to the original author(s) and the source, provide a link to the CreativeCommons licence, and indicate if you modified the licensed material. Youdo not have permission under this licence to share adapted materialderived from this article or parts of it. The images or other third partymaterial in this article are included in the article’s Creative Commonslicence, unless indicated otherwise in a credit line to thematerial. If materialis not included in thearticle’sCreativeCommons licenceandyour intendeduse is not permitted by statutory regulation or exceeds the permitted use,you will need to obtain permission directly from the copyright holder. Toview a copy of this licence, visit http://creativecommons.org/licenses/by-nc-nd/4.0/.© The Author(s) 2024https://doi.org/10.1038/s41699-024-00507-3 Articlenpj 2D Materials and Applications |            (2024) 8:71 9https://doi.org/10.1038/s41699-024-00507-3http://www.nature.com/reprintshttp://creativecommons.org/licenses/by-nc-nd/4.0/http://creativecommons.org/licenses/by-nc-nd/4.0/www.nature.com/npj2dmaterials Achieving nearly barrier free transport in high mobility ReS2 phototransistors with van der Waals contacts Results and discussions Electronic transport Optoelectronic transport Experimental methods Synthesis of ReS2 single crystal Device fabrication Materials and devices characterizations Computational methods Data availability References Acknowledgements Author contributions Competing interests Additional information