The peak position obtained from PL spectrum (Fig. S1 in Supporting information) and the absence of Raman mode (~310 cm
−1) in Raman spectrum (
Fig. 1a) identified the monolayer of the flake [
26]. To explore the transport behavior of pristine and irradiated WSe
2 monolayer, the sample was configured into a back-gated FET device for electrical characterizations. Optical image of FET fabricated from exfoliated WSe
2 monolayer is shown in
Fig. 1a. Subsequently, the device was subjected to electron beam irradiation with variations intensities of fluence.
Fig. 1b displays the transfer characteristics of the as-fabricated WSe
2 FET devices with various irradiation density. The measurements were carried out in vacuum (10
–1 mbar) under a bias of 1 V at room temperature (300K) using Keithley 4200 semiconductor parameter analyzer. The logarithmic plot of transfer characteristics is presented in the inset. The electron mobility can be extracted from the linear regime of transfer plot
via the formula
μ = (
L/
WVdsCi)·(d
Ids/d
Vg), where
L is the channel length,
W is the channel width,
C is the capacitance per unit area between WSe
2 monolayer and back gate given by
C =
ε0εr/
d ≈ 1.15 × 10
−4F/m
2 (
ε0,
εr and
d are the vacuum permittivity, the dielectric constant (~3.9) and thickness of SiO
2 (300 nm), respectively),
Vds = 1 V, and d
Ids/d
Vg represents the slope of the linear region in transfer characteristic.
L and
W of this device are 2.20 and 1.67 μm, respectively. The pristine sample shows typical ambipolar transport behavior because the Fermi level in midgap of pristine WSe
2 facilitates both electron and hole injections [
6]. For pristine WSe
2 monolayer, the on-current of pristine device is 0.079 μA (at
Vds = 1 V,
Vg = 60 V) and the I
ds-V
ds curves show non-linearity and asymmetry (
Fig. 1c and Fig. S2 in Supporting Information), and the electron mobility is calculated to be 0.86 cm
2 V
−1 s
−1. Such poor performances are ascribed to the existence of large Schottky barrier between WSe
2 and Ni electrodes. After irradiation treatment with increasing fluences, the on-current is gradually enhanced, and threshold voltage shifts towards left, as shown in the inset of
Fig. 1b, indicating n-type doping effect on the irradiated WSe
2 FET. The maximal current of 0.83 μA was obtained when the irradiation density was kept at 6.24 × 10
6μm
-2, which is enhanced by an order of magnitude compared to 0.079 μA of the pristine device. However, under higher dosage of irradiation, the current and mobility of the device are reduced due to the introduction of excessive defects. The
Ids−
Vds output characteristic curves at
Vg = 60 V is shown in
Fig. 1c. Similarly, the maximum current also exhibits an increase of an order of magnitude. The
Ids–
Vds curves of pristine and irradiated WSe
2 with increasing gate voltages from -60 V to 60 V are shown in Fig. S2 (Supporting Information). Using the equation [
27]
n = −
C(
Vg –
Vth)/
e, the concentration of electron can be extracted from the transfer plot, where
e is the electronic charge, and
Vth is the threshold voltage that can be obtained by linearly extrapolating transfer curves in the linear regions (
Fig. 1b). The calculated electron mobility and concentration (
Vg = 45 V) as functions of the irradiation density are plotted in
Fig. 1d. The electron mobility significantly improves from 0.86 to 6.66 cm
2 V
−1 s
−1 with irradiation density of 6.24 × 10
6μm
−2. The electron concentration in WSe
2 monolayer calculated at
Vg = 45 V also increases from 1.17 × 10
11cm
−2 to 5.32 × 10
11cm
−2, indicating a considerable injection of electrons. These results confirm the enhancement of n-doping in WSe
2 monolayer. However, the mobility and concentration of electron slightly decrease when the dosage continues to increase, due to the introduction of excessive defects. Remarkably, there is no sharp reduction in electrical performance even though the irradiation dosage increases significantly.