Electric-double-layer p-i-n junctions in WSe2.

While p-n homojunctions in two-dimensional transition metal dichalcogenide materials have been widely reported, few show an ideality factor that is constant over more than a decade in current. In this paper, electric double layer p-i-n junctions in WSe2 are shown with substantially constant ideality factors (2-3) over more than 3 orders of magnitude in current. These lateral junctions use the solid polymer, polyethylene oxide: cesium perchlorate (PEO:CsClO4), to induce degenerate electron and hole carrier densities at the device contacts to form the junction. These high carrier densities aid in reducing the contact resistance and enable the exponential current dependence on voltage to be measured at higher currents than prior reports. Transport measurements of these WSe2 p-i-n homojunctions in combination with COMSOL multiphysics simulations are used to quantify the ion distributions, the semiconductor charge distributions, and the simulated band diagram of these junctions, to allow applications to be more clearly considered.

Pulse Dynamics of Electric Double Layer Formation on All-Solid-State Graphene Field-Effect Transistors.

Electric double layer (EDL) dynamics in graphene field-effect transistors (FETs) gated with polyethylene oxide (PEO)-based electrolytes are studied by molecular dynamics (MD) simulations from picoseconds to nanoseconds and experimentally from microseconds to milliseconds. Under an applied field of approximately mV/nm, EDL formation on graphene FETs gated with PEO:CsClO4 occurs on the timescale of microseconds at room temperature and strengthens within 1 ms to a sheet carrier density of nS ≈ 1013 cm-2. Stronger EDLs (i.e., larger nS) are induced experimentally by pulsing with applied voltages exceeding the electrochemical window of the electrolyte; electrochemistry is avoided using short pulses of a few milliseconds. Dynamics on picosecond to nanosecond timescales are accessed using MD simulations of PEO:LiClO4 between graphene electrodes with field strengths of hundreds of mV/nm which is 100× larger than experiment. At 100 mV/nm, EDL formation initiates in sub-nanoseconds achieving charge densities up to 6 × 1013 cm-2 within 3 nanoseconds. The modeling shows that under sufficiently high electric fields, EDLs with densities ∼1013 cm-2 can form within a nanosecond, which is a timescale relevant for high-performance electronics such as EDL transistors (EDLTs). Moreover, the combination of experiment and modeling shows that the timescale for EDL formation ( nS = 1013 to 1014 cm-2) can be tuned by 9 orders of magnitude by adjusting the field strength by only 3 orders of magnitude.

Monolayer Solid-State Electrolyte for Electric Double Layer Gating of Graphene Field-Effect Transistors.

The electrostatic gating of graphene field-effect transistors is demonstrated using a monolayer electrolyte. The electrolyte, cobalt crown ether phthalocyanine (CoCrPc) and LiClO4, is deposited as a monolayer on the graphene channel, essentially creating an additional two-dimensional layer on top of graphene. The crown ethers on the CoCrPc solvate lithium ions and the ion location is modulated by a backgate without requiring liquid solvent. Ions dope the channel by inducing image charges; the doping level (i.e., induced charge density) can be modulated by the backgate bias with the extent of the surface potential change being controlled by the magnitude and polarity of the backgate bias. With a crown ether to Li+ ratio of 5:1, programming tests for which the backgate is held at -VBG shift the Dirac point by ∼15 V, corresponding to a sheet carrier density on the order of 1012 cm-2. This charge carrier density agrees with the packing density of monolayer CoCrPc on graphene that would be expected with one Li+ for every five crown ethers (at the maximum possible Li+ concentration, 1013 cm-2 is predicted). The crown ethers provide two stable states for the Li+: one near the graphene channel (low-resistance state) and one ∼5 Å away from the channel (high-resistance state). Initial state retention measurements indicate that the two states can be maintained for at least 30 min (maximum time monitored), which is 106 times longer than polymer-based electrolytes at room temperature, with at least a 250 Ω µm difference between the channel resistance in the high- and low-resistance states.

Atomic Layer Deposition of Al2O3 on WSe2 Functionalized by Titanyl Phthalocyanine.

To deposit an ultrathin dielectric onto WSe2, monolayer titanyl phthalocyanine (TiOPc) is deposited by molecular beam epitaxy as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. TiOPc molecules are arranged in a flat monolayer with 4-fold symmetry as measured by scanning tunneling microscopy. ALD pulses of trimethyl aluminum and H2O nucleate on the TiOPc, resulting in a uniform deposition of Al2O3, as confirmed by atomic force microscopy and cross-sectional transmission electron microscopy. The field-effect transistors (FETs) formed using this process have a leakage current of 0.046 pA/µm(2) at 1 V gate bias with 3.0 nm equivalent oxide thickness, which is a lower leakage current than prior reports. The n-branch of the FET yielded a subthreshold swing of 80 mV/decade.

Reconfigurable Ion Gating of 2H-MoTe2 Field-Effect Transistors Using Poly(ethylene oxide)-CsClO4 Solid Polymer Electrolyte.

Transition metal dichalcogenides are relevant for electronic devices owing to their sizable band gaps and absence of dangling bonds on their surfaces. For device development, a controllable method for doping these materials is essential. In this paper, we demonstrate an electrostatic gating method using a solid polymer electrolyte, poly(ethylene oxide) and CsClO4, on exfoliated, multilayer 2H-MoTe2. The electrolyte enables the device to be efficiently reconfigured between n- and p-channel operation with ON/OFF ratios of approximately 5 decades. Sheet carrier densities as high as 1.6 × 10(13) cm(-2) can be achieved because of a large electric double layer capacitance (measured as 4 µF/cm(2)). Further, we show that an in-plane electric field can be used to establish a cation/anion transition region between source and drain, forming a p-n junction in the 2H-MoTe2 channel. This junction is locked in place by decreasing the temperature of the device below the glass transition temperature of the electrolyte. The ideality factor of the p-n junction is 2.3, suggesting that the junction is recombination dominated.

Ultimate thin vertical p-n junction composed of two-dimensional layered molybdenum disulfide.

Semiconducting two-dimensional crystals are currently receiving significant attention because of their great potential to be an ultrathin body for efficient electrostatic modulation, which enables to overcome the limitations of silicon technology. Here we report that, as a key building block for two-dimensional semiconductor devices, vertical p-n junctions are fabricated in ultrathin MoS2 by introducing AuCl3 and benzyl viologen dopants. Unlike usual unipolar MoS2, the MoS2 p-n junctions show ambipolar carrier transport, current rectification via modulation of potential barrier in films thicker than 8 nm and reversed current rectification via tunnelling in films thinner than 8 nm. The ultimate thinness of the vertical p-n homogeneous junctions in MoS2 is experimentally found to be 3 nm, and the chemical doping depth is found to be 1.5 nm. The ultrathin MoS2 p-n junctions present a significant potential of the two-dimensional crystals for flexible, transparent, high-efficiency electronic and optoelectronic applications.

Electronics based on two-dimensional materials.

The compelling demand for higher performance and lower power consumption in electronic systems is the main driving force of the electronics industry's quest for devices and/or architectures based on new materials. Here, we provide a review of electronic devices based on two-dimensional materials, outlining their potential as a technological option beyond scaled complementary metal-oxide-semiconductor switches. We focus on the performance limits and advantages of these materials and associated technologies, when exploited for both digital and analog applications, focusing on the main figures of merit needed to meet industry requirements. We also discuss the use of two-dimensional materials as an enabling factor for flexible electronics and provide our perspectives on future developments.

Direct measurement of Dirac point energy at the graphene/oxide interface.

We report the direct measurement of the Dirac point, the Fermi level, and the work function of graphene by performing internal photoemission measurements on a graphene/SiO(2)/Si structure with a unique optical-cavity enhanced test structure. A complete electronic band alignment at the graphene/SiO(2)/Si interfaces is accurately established. The observation of enhanced photoemission from a one-atom thick graphene layer was possible by taking advantage of the constructive optical interference in the SiO(2) cavity. The photoemission yield was found to follow the well-known linear density-of-states dispersion in the vicinity of the Dirac point. At the flat band condition, the Fermi level was extracted and found to reside 3.3 eV ± 0.05 eV below the bottom of the SiO(2) conduction band. When combined with the shift of the Fermi level from the Dirac point, we are able to ascertain the position of the Dirac point at 3.6 eV ± 0.05 eV with respect to the bottom of the SiO(2) conduction band edge, yielding a work function of 4.5 eV ± 0.05 eV which is in an excellent agreement with theory. The accurate determination of the work function of graphene is of significant importance to the engineering of graphene-based devices, and the measurement technique we have advanced in this Letter will have significant impact on numerous applications for emerging graphene-like 2-dimensional material systems.

The MoS2 Nanotubes with Defect-Controlled Electric Properties.

We describe a two-step synthesis of pure multiwall MoS2 nanotubes with a high degree of homogeneity in size. The Mo6S4I6 nanowires grown directly from elements under temperature gradient conditions in hedgehog-like assemblies were used as precursor material. Transformation in argon-H2S/H2 mixture leads to the MoS2 nanotubes still grouped in hedgehog-like morphology. The described method enables a large-scale production of MoS2 nanotubes and their size control. X-ray diffraction, optical absorption and Raman spectroscopy, scanning electron microscopy with wave dispersive analysis, and transmission electron microscopy were used to characterize the starting Mo6S4I6 nanowires and the MoS2 nanotubes. The unit cell parameters of the Mo6S4I6 phase are proposed. Blue shift in optical absorbance and metallic behavior of MoS2 nanotubes in two-probe measurement are explained by a high defect concentration.