Field Effect Transistor Based on Layered NiPS3

Layered metal phosphochalcogenides of molecular formula, MPX3 (M = Mn, Fe, Co, Ni, etc and X = S, Se) have been emerging as new class of semiconductors towards various catalytic and optoelectronic applications. The low cleavage energy associated with these layered chalcogenides may lead to devices with very thin semiconductor channels. Herein, we report the first successful fabrication of field effect transistor (FET) using layered NiPS3 that reveals n-type semiconducting behavior. Devices using bulk and few-layer NiPS3 with gold contacts show on/off ratios of ~103–105 at 25 °C. The device characteristics reveal an increase in on-state current with decrease in threshold voltage and the Schottky barrier height is extracted to be 112 meV. Density functional theory calculations reveal various parameters that affect electron/hole doping in the layered phosphochalcogenide material.

In the present study, we have synthesized fairly large sized, oriented crystals of NiPS 3 using a high temperature solid state method and explored its use in FETs. The material is thoroughly characterized using X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM) and atomic force microscopy (AFM). Field effect transistors (FETs) have been fabricated and temperature dependent electrical transport measurements have been carried out. Density functional theory (DFT) calculations reveal the possible parameters that help understand the carrier-type observed in the electrical transport studies.

Result and Discussion
Highly oriented crystals of NiPS 3 obtained in the present study are shiny black in colour with high crystallinity and the X-ray diffraction pattern shows very high orientation in the (00 l) direction. Figure 1(a-c) shows the optical images, scanning electron microscopic image with elemental mapping and the XRD pattern of large sized crystals. The Raman spectrum of the bulk material (Fig. 1d) shows a band at 253 cm −1 which is assigned to the A 1g (1) mode. The high intense peak at 176 cm −1 is due E g (2) vibration and the one observed at 384 cm −1 is due to symmetric stretching vibration of P-S bond in the P 2 S 6 units (assigned as A 1g (2) ). The bands at 236 cm −1 , 280 cm −1 , 560 cm −1 and 588 cm −1 are assigned to E g (3) , E g (4) , E g (5) and A 1g (3) modes 40,51,52 . The bulk electrical conductivity has been measured to be 1.64 × 10 −7 S/cm at 25 °C. TMDs such as 2H MoS 2 , MoSe 2 , WSe 2 and MoTe 2 show values of 3 × 10 −2 , 2 × 10 −1 , 6 and 1.8 S/cm respectively at 25 °C 53 .
The trans conductance (g m ) value of NiPS 3 obtained from the fabricated FET devices is 2.5 µS/cm. The TMDs, on the other hand show values of 0.5-3 µS/µm for back-gated devices 16 . The morphology and microstructure of exfoliated NiPS 3 nanosheets are given in Fig. 2. Spatially resolved EDS elemental mapping of Ni, P and S elements reveals a ratio of 1:1:3 as expected with uniform distribution of the elements obtained over the entire surface. The HRTEM image shows well-resolved lattice fringes along (002) plane, confirming the quality of the crystalline nanosheet (Fig. 2b). The selected area electron diffraction (SAED) pattern displays a single set of diffraction spots, further confirming the oriented nature of the exfoliated nanosheets (Fig. 2c).
The low cleavage energy 41 reported for NiPS 3 makes it possible to fabricate devices containing few to several layers of the material. The bulk crystals are exfoliated by mechanical means (Fig. S2) as reported for graphene-based materials 54 . The devices used in the present study are prepared by transferring the material onto pre-fabricated contact pads on highly doped silicon as the back gate with 230 nm SiO 2 dielectric by standard mechanical exfoliation method using scotch tape. The source-drain metal contacts are given using 10/60 nm ITO/Au. The flakes in the devices have been characterized using optical microscopy, Raman spectroscopy (with excitation wave length of 514 nm) and the thickness is measured using atomic force microscopy (AFM). Several micron-sized, large flakeswith thicknesses ranging from 1.5-10 nm have been achieved in the devices. The characteristics discussed the present study are for devices with varying thicknesses of 1.5-60 nm consisting of 2-75 layers. The back gated field effect transistors (FETs) have been fabricated and the characteristics followed. The data given below (Fig. 3) is for a channel length of 2.5 µm. The schematic of the FET along with the optical image, AFM picture and the height profile are shown in Fig. 3. It is clear that a large size single flake is present within the channel and the height is measured to be ~6.5 nm. This corresponds to 6 to 8 layers based on the single layer thickness of around 8 to 11 Å 40 . Standard transistor measurements have been carried out in vacuum (~10 −5 mbar) and to ensure reproducibility, we have carried out measurements using several tens to hundreds of different devices.
The on-off ratio, threshold voltage, carrier type and mobility have been evaluated. The output characteristics of the devices, source-drain voltage (V ds ) vs. source-drain current (I ds ) at different gate voltages are shown in Fig. 4. The non-linear behavior possibly arises due to the schottky barrier contact with Au metal. The current increases with increasing positive gate voltage, suggesting n-type semiconducting behavior. The on-off ratio estimated based onthe transfer characteristics is ~10 3 -10 5 for most of the devices (Fig. 5) at 25 °C. The field effect mobility is extracted from the I ds -V bg curve using the following expression 5 .
where μ is the mobility, W (3 μm) is the channel width, L (2.5 μm) is the channel length and C i (1.5 × 10 −4 F/m 2 ) is the capacitance between the channel and the back gate per unit area (C i = ε o ε r /d; ε o = 8.85 × 10 −12 F⋅m −1 ; ε r = 3.9; d = 230 nm). The mobility values are determined to be ~0.5-1 cm 2 /Vs. It is low as compared to several devices known in the literature. However, it is possible that poor contact with the metal electrode may be responsible as reported for MoS 2 and WSe 2 55,56 . The mobility may be improved further by using high-K dielectric materials in top gated devices and is being presently studied.    A threshold voltage of 10 V is estimated by extrapolating the transfer characteristics. The I on /I off ratio as a function of thickness of the material is shown in Fig. 4d and it is observed that for thicknesses less than 60 nm, the I on /I off ~ 10 5 and it is of the order of 10 4 -10 2 when the thickness is in the range 60-100 nm. Further increase in thickness leads to small I on /I off ratio. This indirectly points to depletion length of ~100 nm for the present device. The I on /I off ratio for MoS 2 has been reported to be in the range of 10 7 -10 1 for thicknesses varying from few nm to 250 nm 57 . The relationship between the depletion length with various parameters of the material is given below (Equation 2), where, k is the Boltzmann constant, T is absolute temperature, q is elementary charge, ε r is the vacuum permittivity, ε s is the relative dielectric constant of NiPS 3 (~9.2) 58 , n i is intrinsic carrier concentration 6 × 10 5 cm −3 , and N d is unintentional doping level. NiPS 3 is known to be an intrinsic semiconductor with low conductivity 59 . For a depletion length of ~100 nm, the N d works out to be 1.5 × 10 17 cm −3 that points to certain level of doping in the material. It should be noted that NiPS 3 possesses [P 2 S 6 ] 4− clusters that leads to 'ionic' interactions with Ni 2+ in the material 60 . The contributions of thermionic emission current component (I thermionic ) and the thermally assisted tunneling current component (I tunneling ) under different gate voltages indicate band bending at the metal -semiconductor interface. The Schottky barrier height (SBH) at the interface has been determined based on thermionic emission model using temperature dependent transport behavior. Figure 5b shows the devicecharacteristics at different temperatures and the change in current with the gate voltages is plotted (Fig. 5c) to extract the SBH using the following relationship (Equation 3).
where A is contact area of the junction, A* is the Richardson constant, q is magnitude of electron charge, Ф B is the Schottky barrier height, k B is Boltzmann constant, n is ideality factor, V ds is drain-source voltage and T is the temperature. For the device shown in Fig. 3, the values of ln(I ds /T 3/2 ) are plotted against 1000/T at different V ds as shown in Fig. 5c. The slope at each bias is determined and plotted as a function of source-drain bias as shown in Fig. 5d. The intercept observed in Fig. 5b is determined (S 0 ). Using the equation 4, the Schottky barrier height is calculated to be 112 meV for the 8 layer device of NiPS 3 . This value is similar to that observed for transition metal chalcogenide based devices 21,61 . Based on the small SBH determined from the transfer behavior, it may be expected that FET would show n-type unipolar behaviour and consequently the barrier height for holes is high [(band gap − SBH) for electrons].

FET devices based on bi-layer and bulk NiPS 3 .
The output characteristics of the bi-layer NiPS 3 device, source-drain voltage (V ds ) vs. source-drain current (I ds ) at different gate voltages are shown in Fig. S3. The current amplitude increases with increasing positive gate voltage as observed for the n-type behavior of eight-layer device. However, the current values for the bi-layer NiPS 3 device are quite low as compared to the multilayer device. It has been reported by Kim and co-workers 62 that density of states for multilayer MoS 2 is several times larger than that of single layer MoS 2 thus making multilayers attractive for device applications with considerable drain currents. The on-off ratio estimated based on the transfer characteristics is ~10 5 for the bi-layer devices and the mobility has been estimated to be ~0.5 cm 2 /Vs. The variation of mobility as a function of thickness of the flake (Fig. S4) is similar to the observations on reported TMD-based devices 63 . This is possibly due to scattering caused by extrinsic charge impurity that decreases as the number of layers increases, as reported for MoS 2 63 . The transport behavior for the device with 60 nm thick flake (Fig. S5) is similar to that observed for other thicknesses. The current values are large with on-off ratio and mobility of ~10 2 -10 3 and ~3.5 cm 2 /Vs respectively. DFT studies. Density functional theory (DFT) calculations have been performed to decipher the electrical transport behavior of NiPS 3 . The crystal structure of NiPS 3 (Fig. 6a) illustrates that the (P 2 S 6 ) 4− units are in staggered configuration where all sulphur atoms are coordinated to Ni in distorted octahedral environment and the van der Waals interlayer distance between S…S is 4.422 Å. The optimized geometry illustrates that the (P 2 S 6 ) 4− units show dihedral (angle involving S-P-P-S structure) angle of 180°. The six S-P-P angles in the (P 2 S 6 ) 4− unit being almost equal (Table S1), reveals that there is no strain present in the P 2 S 6 clusters even when Ni is present in the lattice (Fig. 6b). On the contrary, the NiS 6 10− units clearly reveal distortions in the structure. The distortion parameters (distortion angle, denoted as σ 2 oct) 64-66 are measured as proposed by Robinson et al. 67 . Based on the structure (Fig. 6c), it is seen that there are twelve different bond angles that can be measured and the values are given in the Supporting Information (Table S1). The average σ 2 oct determined for neutral NiPS 3 is 26.02°.
The DOS calculations (Fig. S6a) confirm the presence of up and down spins in the frontier region. The atom projected DOS given in Figure S7 indicates that the 3d bands of Ni (spin-up) are well-mixed with the 3p bands of S confirming strong hybridization as shown in SI (Fig. S6b,d). The upper valence bands are comprised of 3d orbital of Ni and 3p orbital of S. Sulphur dominates the valance band region whereas nickel contribution is maximum at the conduction band region. The orbital projected DOS of individual atoms reveals the presence of large population of 3p orbital of S at the Fermi whereas the contribution by phosphorus is very minimum. This may be due to the strong P-P covalent bond that appears in the lower valance band (−5 to −7 eV) energy region (Fig. S6c). The FET characteristics of transistors and the type of conduction (p-or n-type) will depend on the relative position of the frontier region with respect to the metal contacts. To understand this aspect, the structural modulation of NiPS 3 by electron/hole doping has been studied. The parameters obtained for the optimized geometries of neutral and hole/electron doped (0.1e − per atom) NiPS 3 are given in the supporting information (Table S2 and S3 in SI). Same level of theory has been used to perform calculations in all the cases. Doping an electron (0.1e − per atom) to NiPS 3 results in increased (8.01 eV) van der Waals gap and while for hole doping, the gap is found to be reduced (3.20 eV).
Addition of an electron elongates the Ni-S bond length and the opposite effect is observed by hole doping. Very little changes are observed in P-S and P-P bond lengths during electron and hole doping. Further, it is seen that the addition of electron decreases the bond angle distortion in the NiS 6 octahedra quite considerably to nearly zero (1.5°) while the hole doping increases the distortion angle by 12° from the neutral value of 26.02°. This suggests that electron doping leads to stable octahedral geometry around Ni. The changes observed in NiS 6 units confirm the earlier prediction based on DOS calculations that the doping affects the environment around Ni and S. In the case of electron doping, spin up bands illustrate that the conduction band minimum is observed at G k point and the valance band maximum is found in between L and M k point with a band gap of 1.53 eV while the spin down structure becomes metallic (Fig. 7a). Similarly, the hole doping makes it metallic in nature for spin-up configuration and semiconducting for spin-down configuration (Fig. 7b) with an indirect band gap of 0.47 eV. The results show that NiPS 3 changes from semiconducting to metallic nature tuned by electron/hole doping. Magnitude of band gap may also indirectly indicate the stability of materials upon doping. This can be related to the spin flip-gap related stability on doping (Fig. S1).
The atom projected DOS for individual atoms with and without electron/hole doping are shown in Fig. S7. Ni dominates at the Fermi for electron doping while S dominates for hole doping. Interestingly, for electron doped structure, significant increase in the population of Ni is observed near Fermi as compared to neutral NiPS 3 . It has moved to lower energy region indicating that the structure stabilizes with electron doping. The Ni population decreases near Fermi and minor reduction at the valance band region is observed for the hole doped material. In the case of S, major changes occur only during hole doping. The above results indicate that the electron doping affects Ni and hole doping affects sulphur and consequently the NiS 6 unit. This is supported by the lowered bond angle distortions during electron doping thus leading to stable NiS 6 10− octahedral environment.

Summary.
The present study has shown the possibilities of fabricating field effect transistors using layered phosphochalcogenides, NiPS 3 . The FET characteristics show n-type behavior with on/off ratio of 10 3 -10 5 . The DFT studies have predicted the transport characteristics and are experimentally verified. The phosphochalcogenides with magnetically active centers such as Ni, Co and Mn open ways to flip the spin behavior under magnetic field.

Experimental Section
Synthesis of NiPS 3 crystals. Single crystals of NiPS 3 have been synthesized by chemical vapor transport (CVT) technique using iodine as the transporting agent. Pure elements (99.99%) of nickel, phosphorous and sulphur (Aldrich) in stoichiometric proportions with I 2 (2 mg/cc) were sealed in an evacuated quartz ampoule. After several attempts, the optimum reaction conditions were arrived at, with hot zone temperature of 950 °C and cold zone of 850 °C that result in high quality, large sized crystals of NiPS 3 . The schematics of CVT growth setup and the parameters used are given in the supporting information (Fig. S8, S9).
Characterization. The physicochemical nature of the crystals were identified using X-ray diffraction (XRD)   6 10− unit with different bond angles. Blue, pink and brown colours represent nickel, phosphorus and sulphur atoms respectively. constant, 4.5 N/m). The samples for TEM measurements were prepared by dispersing few-layer NiPS 3 colloids onto carbon-coated copper grid and dried under vacuum. The electrical measurements were performed under high vacuum (<5 × 10 −5 ) using Agilent B1500 semiconductor parametric analyzer. Devices were prepared by transferring the material onto pre-fabricated contact pads (Fraunhofer IPMS, Germany) on highly doped silicon with 230 nm SiO 2 dielectric using standard mechanical exfoliation method. Source-drain metal contacts were given using 10/60 nm ITO/Au. Highly doped silicon act as the back gate. It was observed that the lithography procedure wherein the sample was exfoliated on Si/SiO 2 surface and subsequently depositing the contact pads on the pre-marked areas led to sample deterioration due to the use of solvents during the process.
Computational methodology. Geometrical optimization calculations for all extended structures has been performed using VASP code 68,69 with plane wave basis truncated at a kinetic energy of 500 eV. The projector augmented wave (PAW) scheme as incorporated in the Vienna ab initio simulation package (VASP) is used in the study (Supporting Information). Density of states and band structure calculations have been performed followed by optimization using CASTEP package 70 including LDA + U approximation. Calculations have been performed for neutral NiPS 3 and electron/hole doping separately, and the values are tabulated in Table S1 (Supporting Information).