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 × 1013 cm–2 can be achieved because of a large electric double layer capacitance (measured as 4 μF/cm2). 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.
The effect of hole localization on photocatalytic activity of Pt-tipped semiconductor nanocrystals is investigated. By tuning the energy balance at the semiconductor–ligand interface, we demonstrate ...that hydrogen production on Pt sites is efficient only when electron-donating molecules are used for stabilizing semiconductor surfaces. These surfactants play an important role in enabling an efficient and stable reduction of water by heterostructured nanocrystals as they fill vacancies in the valence band of the semiconductor domain, preventing its degradation. In particular, we show that the energy of oxidizing holes can be efficiently transferred to a ligand moiety, leaving the semiconductor domain intact. This allows reusing the inorganic portion of the “degraded” nanocrystal-ligand system simply by recharging these nanoparticles with fresh ligands.
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 −V BG 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.
Poly(vinyl alcohol) (PVA) and LiClO4, a solid polymer electrolyte with a glass transition temperature (T g) of 80 °C, is used to electrostatically gate graphene field-effect transistors. The ions in ...PVA:LiClO4 are drifted into place by field-effect at T > T g, providing n- or p-type doping, and when the device is cooled to room temperature, the polymer mobility and, hence ion mobility are arrested and the electric double layer (EDL) is “locked” into place in the absence of a gate bias. Unlike other electrolytes used to gate two-dimensional devices for which the T g, and therefore the “locking” temperature, is well below room temperature, the electrolyte demonstrated in this work provides a route to achieve room-temperature EDL stability. Specifically, a 6 orders of magnitude increase in the room temperature EDL retention time is demonstrated over the commonly used electrolyte, poly(ethylene oxide) (PEO) and LiClO4. Hall measurements confirm that large sheet carrier densities can be achieved with PVA:LiClO4 at top gate programming voltages of ±2 V (−6.3 ± 0.03 × 1013 cm–2 for electrons and 1.6 ± 0.3 × 1014 cm–2 for holes). Transient drain current measurements show that at least 75% of the EDL is retained after more than 4 h at room temperature. Unlike PEO-based electrolytes, PVA:LiClO4 is compatible with the chemicals used in standard photolithographic processes enabling the direct deposition of patterned, metal contacts on the surface of the electrolyte. A thermal instability in the electrolyte is detected by both I–V measurements and differential scanning calorimetry, and FTIR measurements suggest that thermally catalyzed cross-linking may be driving phase separation between the polymer and the salt. Nevertheless, this work highlights how the relationship between polymer and ion mobility can be exploited to tune the state retention time and the charge carrier density of a 2D crystal transistor.
A general strategy for low-temperature processing of colloidal nanocrystals into all-inorganic films is reported. The present methodology goes beyond the traditional ligand-interlinking scheme and ...relies on encapsulation of morphologically defined nanocrystal arrays into a matrix of a wide-band gap semiconductor, which preserves optoelectronic properties of individual nanoparticles while rendering the nanocrystal film photoconductive. Fabricated solids exhibit excellent thermal stability, which is attributed to the heteroepitaxial structure of nanocrystal–matrix interfaces, and show compelling light-harvesting performance in prototype solar cells.
Two-dimensional (2D) materials, particularly graphene and transition metal dichalcogenides (TMDs), have been the subject of many recent studies because of their unique mechanical and electrical ...properties. All the bonds are in-plane, and the layers are separated by a van der Waals gap. While most properties can be characterized by measuring the unmodified materials, many electrical properties require the charge state of the material to be modulated. For traditional semiconductors, this is achieved by field-effect gating through a dielectric combined with substitutional doping. However, the lack of dangling surface bonds makes the deposition of a gate dielectric challenging, and methods for controlled substitutional doping have not yet been developed. Solid polymer electrolytes (SPEs), which consist of a salt dissolved in a polymer, offer a relatively simple solution. By using the SPE in the place of a gate dielectric, ions in the SPE are driven to close proximity (∼1 nm) of the material surface to induce image charges in the 2D material. The ion and corresponding image charge layers are collectively known as an electrostatic (or electric) double layer (EDL). The EDL can induce charge carrier densities in the material in excess of 1014 cm–2. The work presented in this dissertation details ways of using EDL-induced sheet charges to open up new applications in the field of nanoelectronics. By tuning the glass transition temperature of the SPE, the EDL retention time can be increased by six orders of magnitude at room temperature while still achieving charge carrier densities in excess of 1014 cm –2, enabling unique hardware security. We also investigate applying large electric fields over the SPE, which causes volumetric change in 2D electronics due to ion intercalation. 2D devices typically are top-gated by the SPE. However, in this work, backgating by an SPE is demonstrated for the first time. As well, we explore partial switching of the ferroelectric material HfZrO 2 The objective is to use the ferroelectric in conjunction with a 2D channel to form a two-terminal resistive processing unit for use in hardware-based neural networks.
The present study investigates the photocatalytic activity of ZnSe/CdS core/shell semiconductor nanocrystals. These nanoparticles exhibit a spatial separation of photoinduced charges between the core ...and the shell domains, which makes them potentially viable for photocatalytic applications. Unfortunately, one of the excited charges remains inside the core semiconductor and thus cannot efficiently react with the external environment. Here, we explore this issue by investigating the mechanisms of hole extraction from the ZnSe core to the surface of the CdS shell. In particular, the effect of shell thickness in ZnSe/CdS core/shell nanocrystals on the ability of core-localized charges to perform oxidative reactions was determined. By using a combination of time-resolved spectroscopy and electrochemical techniques, we demonstrate that the use of hole-scavenging surfactants facilitates an efficient transfer of core-localized holes to the surface even in the case of shells exceeding 7 nm in thickness. These measurements further demonstrate that photoinduced holes can be extracted from the core faster than they recombine with shell-localized electrons, indicating that most of the absorbed energy in ZnSe/CdS nanocrystals can be used to drive catalytic reactions.
Gokmen and Vlasov proposed that the training of deep neural networks would be dramatically accelerated by the realization of resistive processing units that can store analog weights to minimize data ...movement during training 1. Here we measure and evaluate the partial switching of the ferroelectric (FE) PbZrTiO 3 (PZT) and, for the first time, Hf 0.8 Zr 0.2 O 2 , (HZO), formed by atomic layer deposition, for use as a nonvolatile analog memory element. The spontaneous polarization P of a FE can be switched by applying a voltage that exceeds the coercive voltage for switching of the FE. This polarization, when fully switched, can have a value of +P or -P (C/cm 2 ), depending on the polarity of the applied voltage. Ferroelectric memory is typically operated in fully switched +P or -P polarizations, however with short (<; 10 μs) pulsed voltages, the FE can be partially switched, i.e. only a fraction of the domains are switched during the pulse duration as shown by Tokumitsu in PZT 2 and indicated in Fig. 1.
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