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1.
Nano Lett ; 19(12): 8911-8919, 2019 12 11.
Artigo em Inglês | MEDLINE | ID: mdl-31661286

RESUMO

A molecularly thin electrolyte is developed to demonstrate a nonvolatile, solid-state, one-transistor (1T) memory based on an electric-double-layer (EDL) gated WSe2 field-effect transistor (FET). The custom-designed monolayer electrolyte consists of cobalt crown ether phthalocyanine and lithium ions, which are positioned by field-effect at either the surface of the WSe2 channel or an h-BN capping layer to achieve "1" or "0", respectively. Bistability in the monolayer electrolyte memory is significantly improved by the h-BN cap with density functional theory (DFT) calculations showing enhanced trapping of Li+ near h-BN due to a ∼1.34 eV increase in the absolute value of the adsorption energy compared to vacuum. The threshold voltage shift between the two states corresponds to a change in charge density of ∼2.5 × 1012 cm-2, and an On/Off ratio exceeding 104 at a back gate voltage of 0 V. The On/Off ratio remains stable after 1000 cycles and the retention time for each state exceeds 6 h (max measured). When the write time approaches 1 ms, the On/Off ratio remains >102, showing that the monolayer electrolyte-gated FET can respond on time scales similar to existing flash memory. The data suggest that faster switching times and lower switching voltages could be feasible by top gating.

2.
ACS Appl Mater Interfaces ; 11(39): 35879-35887, 2019 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-31486629

RESUMO

Electric double-layer (EDL) gating using a custom-synthesized polyester single-ion conductor (PE400-Li) is demonstrated on two-dimensional (2D) crystals for the first time. The electronic properties of graphene and MoTe2 field-effect transistors (FETs) gated with the single-ion conductor are directly compared to a poly(ethylene oxide) dual-ion conductor (PEO:CsClO4). The anions in the single-ion conductor are covalently bound to the backbone of the polymer, leaving only the cations free to form an EDL at the negative electrode and a corresponding cationic depletion layer at the positive electrode. Because the cations are mobile in both the single- and dual-ion conductors, a similar enhancement of the n-branch is observed in both graphene and MoTe2. Specifically, the single-ion conductor decreases the subthreshold swing in the n-branch of the bare MoTe2 FET from 5000 to 250 mV/dec and increases the current density and on/off ratio by two orders of magnitude. However, the single-ion conductor suppressed the p-branch in both the graphene and the MoTe2 FETs, and finite element modeling of ion transport shows that this result is unique to single-ion conductor gating in combination with an asymmetric gate/channel geometry. Both the experiments and modeling suggest that single-ion conductor-gated FETs can achieve sheet densities up to 1014 cm-2, which corresponds to a charge density that would theoretically be sufficient to induce several percent strain in monolayer 2D crystals and potentially induce a semiconductor-to-metal phase transition in MoTe2.

3.
Front Chem ; 7: 216, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31024900

RESUMO

In situ fabrication of nanostructures within a solid-polymer electrolyte confined to subwavelength-diameter nanoapertures is a promising approach for producing nanomaterials for nanophotonic and chemical sensing applications. The solid-polymer electrolyte can be patterned by lithographic photopolymerization of poly(ethylene glycol) diacrylate (PEGDA)-based silver cation (Ag+)-containing polyelectrolyte. Here, we present a new method for fabricating nanopore-templated Ag nanoparticle (AgNP) arrays by in situ photopolymerization using a zero-mode waveguide (ZMW) array to simultaneously template embedded AgNPs and control the spatial distribution of the optical field used for photopolymerization. The approach starts with an array of nanopores fabricated by sequential layer-by-layer deposition and focused ion beam milling. These structures have an optically transparent bottom, allowing access of the optical radiation to the attoliter-volume ZMW region to photopolymerize a PEGDA monomer solution containing AgNPs and Ag+. The electric field intensity distribution is calculated for various ZMW optical cladding layer thicknesses using finite-element simulations, closely following the light-blocking efficiency of the optical cladding layer. The fidelity of the polyelectrolyte nanopillar pattern was optimized with respect to experimental conditions, including the presence or absence of Ag+ and AgNPs and the concentrations of PEGDA and Ag+. The self-templated approach for photopatterning high-resolution photolabile polyelectrolyte nanostructures directly within a ZMW array could lead to a new class of metamaterials formed by embedding metal nanoparticles within a dielectric in a well-defined spatial array.

4.
ACS Appl Mater Interfaces ; 10(49): 43166-43176, 2018 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-30422628

RESUMO

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.

5.
ACS Appl Mater Interfaces ; 10(47): 40831-40837, 2018 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-30384598

RESUMO

The utilization of alkali salts, such as NaCl and KI, has enabled the successful growth of large single domain and fully coalesced polycrystalline two-dimensional (2D) transition-metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties is not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkali-assisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in as-grown, large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and high density of nanoscale MoS2 particles (4 ± 0.7/µm2). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ∼20× and degrades transistor performance.

6.
Small ; 14(39): e1802023, 2018 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-30118585

RESUMO

Materials with reconfigurable optical properties are candidates for applications such as optical cloaking and wearable sensors. One approach to fabricate these materials is to use external fields to form and dissolve nanoscale conductive channels in well-defined locations within a polymer. In this study, conductive atomic force microscopy is used to electrochemically form and dissolve nanoscale conductive filaments at spatially distinct points in a polyethylene glycol diacrylate (PEGDA)-based electrolyte blended with varying amounts of ionic liquid (IL) and silver salt. The fastest filament formation and dissolution times are detected in a PEGDA/IL composite that has the largest modulus (several GPa) and the highest polymer crystal fraction. This is unexpected because filament formation and dissolution events are controlled by ion transport, which is typically faster within amorphous regions where polymer mobility is high. Filament kinetics in primarily amorphous and crystalline regions are measured, and two different mechanisms are observed. The formation time distributions show a power-law dependence in the crystalline regions, attributable to hopping-based ion transport, while amorphous regions show a normal distribution. The results indicate that the timescale of filament formation/dissolution is determined by local structure, and suggest that structure could be used to tune the optical properties of the film.

7.
ACS Nano ; 12(2): 965-975, 2018 02 27.
Artigo em Inglês | MEDLINE | ID: mdl-29360349

RESUMO

Atomically thin transition metal dichalcogenides (TMDs) are of interest for next-generation electronics and optoelectronics. Here, we demonstrate device-ready synthetic tungsten diselenide (WSe2) via metal-organic chemical vapor deposition and provide key insights into the phenomena that control the properties of large-area, epitaxial TMDs. When epitaxy is achieved, the sapphire surface reconstructs, leading to strong 2D/3D (i.e., TMD/substrate) interactions that impact carrier transport. Furthermore, we demonstrate that substrate step edges are a major source of carrier doping and scattering. Even with 2D/3D coupling, transistors utilizing transfer-free epitaxial WSe2/sapphire exhibit ambipolar behavior with excellent on/off ratios (∼107), high current density (1-10 µA·µm-1), and good field-effect transistor mobility (∼30 cm2·V-1·s-1) at room temperature. This work establishes that realization of electronic-grade epitaxial TMDs must consider the impact of the TMD precursors, substrate, and the 2D/3D interface as leading factors in electronic performance.

8.
Sci Rep ; 7(1): 16938, 2017 12 05.
Artigo em Inglês | MEDLINE | ID: mdl-29209000

RESUMO

Evaluating and tuning the properties of two-dimensional (2D) materials is a major focus of advancing 2D science and technology. While many claim that the photonic properties of a 2D layer provide evidence that the material is "high quality", this may not be true for electronic performance. In this work, we deconvolute the photonic and electronic response of synthetic monolayer molybdenum disulfide. We demonstrate that enhanced photoluminescence can be robustly engineered via the proper choice of substrate, where growth of MoS2 on r-plane sapphire can yield >100x enhancement in PL and carrier lifetime due to increased molybdenum-oxygen bonding compared to that of traditionally grown MoS2 on c-plane sapphire. These dramatic enhancements in optical properties are similar to those of super-acid treated MoS2, and suggest that the electronic properties of the MoS2 are also superior. However, a direct comparison of the charge transport properties indicates that the enhanced PL due to increased Mo-O bonding leads to p-type compensation doping, and is accompanied by a 2x degradation in transport properties compared to MoS2 grown on c-plane sapphire. This work provides a foundation for understanding the link between photonic and electronic performance of 2D semiconducting layers, and demonstrates that they are not always correlated.

9.
ACS Appl Mater Interfaces ; 9(29): 25006-25013, 2017 Jul 26.
Artigo em Inglês | MEDLINE | ID: mdl-28715196

RESUMO

Poly(vinyl alcohol) (PVA) and LiClO4, a solid polymer electrolyte with a glass transition temperature (Tg) 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 > Tg, 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 Tg, 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.

10.
ACS Nano ; 11(5): 4976-4984, 2017 05 23.
Artigo em Inglês | MEDLINE | ID: mdl-28459548

RESUMO

Nanoscale conductive filaments, usually associated with resistive memory or memristor technology, may also be used for chemical sensing and nanophotonic applications; however, realistic implementation of the technology requires precise knowledge of the conditions that control the formation and dissolution of filaments. Here we describe and characterize an addressable direct-write nanoelectrochemical approach to achieve repeatable formation/dissolution of Ag filaments across a ∼100 nm poly(ethylene oxide) (PEO) film containing either Ag+ alone or Ag+ together with 50 nm Ag-nanoparticles acting as bipolar electrodes. Using a conductive AFM tip, formation occurs when the PEO film is subjected to a forward bias, and dissolution occurs under reverse bias. Formation-dissolution kinetics were studied for three film compositions: Ag|PEO-Ag+, Ag|poly(ethylene glycol) monolayer-PEO-Ag+, and Ag|poly(ethylene glycol) monolayer-PEO-Ag+/Ag-nanoparticle. Statistical analysis shows that the distribution of formation times exhibits Gaussian behavior, and the fastest average initial formation time occurs for the Ag|PEO-Ag+ system. In contrast, formation in the presence of Ag nanoparticles likely proceeds by a noncontact bipolar electrochemical mechanism, exhibiting the slowest initial filament formation. Dissolution times are log-normal for all three systems, and repeated reformation of filaments from previously formed structures is characterized by rapid regrowth. The direct-write bipolar electrochemical deposition/dissolution strategy developed here presents an approach to reconfigurable, noncontact in situ wiring of nanoparticle arrays-thereby enabling applications where actively controlled connectivity of nanoparticle arrays is used to manipulate nanoelectronic and nanophotonic behavior. The system further allows for facile manipulation of experimental conditions while simultaneously characterizing surface conditions and filament formation/dissolution kinetics.

11.
ACS Nano ; 11(6): 5453-5464, 2017 06 27.
Artigo em Inglês | MEDLINE | ID: mdl-28511001

RESUMO

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.

12.
ACS Nano ; 10(7): 6888-96, 2016 07 26.
Artigo em Inglês | MEDLINE | ID: mdl-27305595

RESUMO

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.

13.
ACS Nano ; 10(4): 4258-67, 2016 04 26.
Artigo em Inglês | MEDLINE | ID: mdl-26991824

RESUMO

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe2 terraces, air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe2 monolayers. The preferential oxidation at the WSe2 edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became about 1.08 eV larger than that of the WSe2 terraces, resulting in the electronic passivation of the WSe2.

14.
Nano Lett ; 15(10): 6626-33, 2015 Oct 14.
Artigo em Inglês | MEDLINE | ID: mdl-26393281

RESUMO

Several proposed beyond-CMOS devices based on two-dimensional (2D) heterostructures require the deposition of thin dielectrics between 2D layers. However, the direct deposition of dielectrics on 2D materials is challenging due to their inert surface chemistry. To deposit high-quality, thin dielectrics on 2D materials, a flat lying titanyl phthalocyanine (TiOPc) monolayer, deposited via the molecular beam epitaxy, was employed to create a seed layer for atomic layer deposition (ALD) on 2D materials, and the initial stage of growth was probed using in situ STM. ALD pulses of trimethyl aluminum (TMA) and H2O resulted in the uniform deposition of AlOx on the TiOPc/HOPG. The uniformity of the dielectric is consistent with DFT calculations showing multiple reaction sites are available on the TiOPc molecule for reaction with TMA. Capacitors prepared with 50 cycles of AlOx on TiOPc/graphene display a capacitance greater than 1000 nF/cm(2), and dual-gated devices have current densities of 10(-7)A/cm(2) with 40 cycles.

15.
ACS Nano ; 9(5): 4900-10, 2015 May 26.
Artigo em Inglês | MEDLINE | ID: mdl-25877681

RESUMO

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.

16.
Langmuir ; 29(10): 3259-68, 2013 Mar 12.
Artigo em Inglês | MEDLINE | ID: mdl-23441753

RESUMO

The structure of a hydrated poly(N-isopropylacrylamide) brush loaded with 5 vol % Isoniazid is studied as a function of temperature using neutron reflectometry (NR) and atomic force microscopy (AFM). NR measurements show that Isoniazid increases the thickness of the brush before, during and after the polymer collapse, and it is retained inside the brush at all measured temperatures. The Isoniazid concentration in the expanded brush is ~14% higher than in the bulk solution, and the concentration nearly doubles in the collapsed polymer, suggesting stronger binding between Isoniazid and the polymer compared to water, even at temperatures below the lower critical solution temperature (LCST) where the polymer is hydrophilic. Typically, additives that bind strongly to the polymer backbone and increase the hydrophilicity of the polymer will delay the onset of the LCST, which is suggested by AFM and NR measurements. The extent of small-molecule loading and distribution throughout a thermo-responsive polymer brush, such as pNIPAAm, will have important consequences for applications such as drug delivery and gating.


Assuntos
Acrilamidas/química , Microscopia de Força Atômica/métodos , Polímeros/química , Resinas Acrílicas , Portadores de Fármacos/química , Isoniazida/química
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