Amorphous to Crystal Phase Change Memory Effect with Two-Fold Bandgap Difference in Semiconducting K2Bi8Se13.

Chalcogenide-based phase change memory (PCM) is a key enabling technology for optical data storage and electrical nonvolatile memory. Here, we report a new phase change chalcogenide consisting of a 3D network of ionic (K···Se) and covalent bonds (Bi-Se), K2Bi8Se13 (KBS). Thin films of amorphous KBS deposited by DC sputtering are structurally and chemically homogeneous and exhibit a surface roughness of 5 nm. The KBS film crystallizes upon heating at ∼483 K. The optical bandgap of the amorphous film is about 1.25 eV, while its crystalline phase has a bandgap of ∼0.65 eV shows 2-fold difference between the two states. The bulk electrical conductivity of the amorphous and crystalline film is ∼7.5 × 10-4 and ∼2.7 × 10-2 S/cm, respectively. We have demonstrated a phase change memory effect in KBS by Joule heating in a technologically relevant vertical memory cell architecture. Upon Joule heating, the vertical device undergoes switching from its amorphous to crystalline state of KBS at 1-1.5 V (∼50 kV/cm), increasing conductivity by a factor of ∼40. Besides the large electrical and optical contrast in the crystalline and amorphous KBS, its elemental cost-effectiveness, stoichiometry, fast crystallization kinetics, as determined by the ratio of the glass transition and melting temperature, Tg/Tm ∼ 0.5, as well as the scalable synthesis of the thin film determine that KBS is a promising PC material for next general phase change memory.

Molecular-Scale Characterization of Photoinduced Charge Separation in Mixed-Dimensional InSe-Organic van der Waals Heterostructures.

Layered indium selenide (InSe) is an emerging two-dimensional semiconductor that has shown significant promise for high-performance transistors and photodetectors. The range of optoelectronic applications for InSe can potentially be broadened by forming mixed-dimensional van der Waals heterostructures with zero-dimensional molecular systems that are widely employed in organic electronics and photovoltaics. Here, we report the spatially resolved investigation of photoinduced charge separation between InSe and two molecules (C70 and C8-BTBT) using scanning tunneling microscopy combined with laser illumination. We experimentally and computationally show that InSe forms type-II and type-I heterojunctions with C70 and C8-BTBT, respectively, due to an interplay of charge transfer and dielectric screening at the interface. Laser-excited scanning tunneling spectroscopy reveals a ∼0.25 eV decrease in the energy of the lowest unoccupied molecular orbital of C70 with optical illumination. Furthermore, photoluminescence spectroscopy and Kelvin probe force microscopy indicate that electron transfer from InSe to C70 in the type-II heterojunction induces a photovoltage that quantitatively matches the observed downshift in the tunneling spectra. In contrast, no significant changes are observed upon optical illumination in the type-I heterojunction formed between InSe and C8-BTBT. Density functional theory calculations further show that, despite the weak coupling between the molecular species and InSe, the band alignment of these mixed-dimensional heterostructures strongly differs from the one suggested by the ionization potential and electronic affinities of the isolated components. Self-energy-corrected density functional theory indicates that these effects are the result of the combination of charge redistribution at the interface and heterogeneous dielectric screening of the electron-electron interactions in the heterostructure. In addition to providing specific insight for mixed-dimensional InSe-organic van der Waals heterostructures, this work establishes a general experimental methodology for studying localized charge transfer at the molecular scale that is applicable to other photoactive nanoscale systems.

Suppressing Ambient Degradation of Exfoliated InSe Nanosheet Devices via Seeded Atomic Layer Deposition Encapsulation.

With exceptional charge carrier mobilities and a direct bandgap at most thicknesses, indium selenide (InSe) is an emerging layered semiconductor that has generated significant interest for electronic and optoelectronic applications. However, exfoliated InSe nanosheets are susceptible to rapid degradation in ambient conditions, thus limiting their technological potential. In addition to morphological changes upon ambient exposure, the mobilities and current modulation on/off ratios of InSe transistors, as well as the responsivities of InSe photodetectors, decrease by over 3 orders of magnitude within 12 h of ambient exposure. In an effort to mitigate these deleterious effects, here we present an encapsulation scheme based on seeded atomic layer deposition that provides pinhole-free growth of alumina without compromising the intrinsic electronic properties of the underlying InSe. In particular, this encapsulation provides reproducible InSe field-effect transistor characteristics and InSe photodetector responsivities in excess of 107 A/W following ambient exposure for time periods on the order of months. Because atomic layer deposition is a highly scalable and manufacturable process, this work will accelerate ongoing efforts to integrate InSe nanosheets into electronic and optoelectronic technologies.

Solution-Based Processing of Optoelectronically Active Indium Selenide.

Layered indium selenide (InSe) presents unique properties for high-performance electronic and optoelectronic device applications. However, efforts to process InSe using traditional liquid phase exfoliation methods based on surfactant-assisted aqueous dispersions or organic solvents with high boiling points compromise electronic properties due to residual surface contamination and chemical degradation. Here, these limitations are overcome by utilizing a surfactant-free, low boiling point, deoxygenated cosolvent system. The resulting InSe flakes and thin films possess minimal processing residues and are structurally and chemically pristine. When employed in photodetectors, individual InSe nanosheets exhibit a maximum photoresponsivity of ≈5 × 107 A W-1 , which is the highest value of any solution-processed monolithic semiconductor to date. Furthermore, the surfactant-free cosolvent system not only stabilizes InSe dispersions but is also amenable to the assembly of electronically percolating InSe flake arrays without posttreatment, which enables the realization of ultrahigh performance thin-film photodetectors. This surfactant-free, deoxygenated cosolvent approach can be generalized to other layered materials, thereby presenting additional opportunities for solution-processed thin-film electronic and optoelectronic technologies.

Extrinsic polarization-controlled optical anisotropy in plasmon-black phosphorus coupled system.

Two-dimensional black phosphorus (BP) has drawn extensive research interest due to its promising anisotropic photonic and electronic properties. Here, we study anisotropic optical absorption and photoresponse of exfoliated BP flakes at visible frequencies. We enhance this intrinsic optical anisotropy in BP flakes by coupling plasmonic rectangular nanopatch arrays that support localized surface plasmon resonances. In particular, by combining extrinsic anisotropic plasmonic nanostructures lithographically aligned with intrinsically anisotropic BP flakes, we demonstrate for the first time a combined anisotropic plasmonic-semiconductor coupling that provides significant control over the polarization-dependent optical properties of the plasmon-BP hybrid material system, enhancing polarization-sensitive responses to a larger degree. This hybrid material system not only unveils the plasmon-enhanced mechanisms in BP, but also provides novel controllable functionalities in optoelectronic device applications involving polarization-sensitive optical and electrical responses.

All-Electrical Determination of Crystal Orientation in Anisotropic Two-Dimensional Materials.

The crystal orientation of an exfoliated black phosphorous flake is determined by purely electrical means. A sequence of three resistance measurements on an arbitrarily shaped flake with five contacts determines the three independent components of the anisotropic in-plane resistivity tensor, thereby revealing the crystal axes. The resistivity anisotropy ratio decreases linearly with increasing temperature T and carrier density reaching a maximum ratio of 3.0 at low temperatures and densities, while mobility indicates impurity scattering at low T and acoustic phonon scattering at high T.

Scanning Probe Nanopatterning and Layer-by-Layer Thinning of Black Phosphorus.

Nanopatterning and layer-by-layer thinning of black phosphorus is demonstrated with conductive atomic-force-microscope anodic oxidation. The liquid-phase patterning byproduct is readily removed by water rinsing. An alternating-current bias enables direct nanopatterning and thinning on insulating substrates such as SiO2 /Si. Field-effect transistors with patterned channels show significant improvements in current modulation by up to a factor of 50.

Covalent functionalization and passivation of exfoliated black phosphorus via aryl diazonium chemistry.

Functionalization of atomically thin nanomaterials enables the tailoring of their chemical, optical and electronic properties. Exfoliated black phosphorus (BP)-a layered two-dimensional semiconductor-exhibits favourable charge-carrier mobility, tunable bandgap and highly anisotropic properties, but it is chemically reactive and degrades rapidly in ambient conditions. Here we show that covalent aryl diazonium functionalization suppresses the chemical degradation of exfoliated BP even after three weeks of ambient exposure. This chemical modification scheme spontaneously forms phosphorus-carbon bonds, has a reaction rate sensitive to the aryl diazonium substituent and alters the electronic properties of exfoliated BP, ultimately yielding a strong, tunable p-type doping that simultaneously improves the field-effect transistor mobility and on/off current ratio. This chemical functionalization pathway controllably modifies the properties of exfoliated BP, and thus improves its prospects for nanoelectronic applications.

Stable aqueous dispersions of optically and electronically active phosphorene.

Understanding and exploiting the remarkable optical and electronic properties of phosphorene require mass production methods that avoid chemical degradation. Although solution-based strategies have been developed for scalable exfoliation of black phosphorus, these techniques have thus far used anhydrous organic solvents in an effort to minimize exposure to known oxidants, but at the cost of limited exfoliation yield and flake size distribution. Here, we present an alternative phosphorene production method based on surfactant-assisted exfoliation and postprocessing of black phosphorus in deoxygenated water. From comprehensive microscopic and spectroscopic analysis, this approach is shown to yield phosphorene dispersions that are stable, highly concentrated, and comparable to micromechanically exfoliated phosphorene in structure and chemistry. Due to the high exfoliation efficiency of this process, the resulting phosphorene flakes are thinner than anhydrous organic solvent dispersions, thus allowing the observation of layer-dependent photoluminescence down to the monolayer limit. Furthermore, to demonstrate preservation of electronic properties following solution processing, the aqueous-exfoliated phosphorene flakes are used in field-effect transistors with high drive currents and current modulation ratios. Overall, this method enables the isolation and mass production of few-layer phosphorene, which will accelerate ongoing efforts to realize a diverse range of phosphorene-based applications.

Chemically Tailoring Semiconducting Two-Dimensional Transition Metal Dichalcogenides and Black Phosphorus.

Two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) and black phosphorus (BP) have beneficial electronic, optical, and physical properties at the few-layer limit. As atomically thin materials, 2D TMDCs and BP are highly sensitive to their environment and chemical modification, resulting in a strong dependence of their properties on substrate effects, intrinsic defects, and extrinsic adsorbates. Furthermore, the integration of 2D semiconductors into electronic and optoelectronic devices introduces unique challenges at metal-semiconductor and dielectric-semiconductor interfaces. Here, we review emerging efforts to understand and exploit chemical effects to influence the properties of 2D TMDCs and BP. In some cases, surface chemistry leads to significant degradation, thus necessitating the development of robust passivation schemes. On the other hand, appropriately designed chemical modification can be used to beneficially tailor electronic properties, such as controlling doping levels and charge carrier concentrations. Overall, chemical methods allow substantial tunability of the properties of 2D TMDCs and BP, thereby enabling significant future opportunities to optimize performance for device applications.

Probing Out-of-Plane Charge Transport in Black Phosphorus with Graphene-Contacted Vertical Field-Effect Transistors.

Black phosphorus (BP) has recently emerged as a promising narrow band gap layered semiconductor with optoelectronic properties that bridge the gap between semimetallic graphene and wide band gap transition metal dichalcogenides such as MoS2. To date, BP field-effect transistors have utilized a lateral geometry with in-plane transport dominating device characteristics. In contrast, we present here a vertical field-effect transistor geometry based on a graphene/BP van der Waals heterostructure. The resulting device characteristics include high on-state current densities (>1600 A/cm(2)) and current on/off ratios exceeding 800 at low temperature. Two distinct charge transport mechanisms are identified, which are dominant for different regimes of temperature and gate voltage. In particular, the Schottky barrier between graphene and BP determines charge transport at high temperatures and positive gate voltages, whereas tunneling dominates at low temperatures and negative gate voltages. These results elucidate out-of-plane electronic transport in BP and thus have implications for the design and operation of BP-based van der Waals heterostructures.

Ultrahigh sensitivity and layer-dependent sensing performance of phosphorene-based gas sensors.

Two-dimensional (2D) layered materials have attracted significant attention for device applications because of their unique structures and outstanding properties. Here, a field-effect transistor (FET) sensor device is fabricated based on 2D phosphorene nanosheets (PNSs). The PNS sensor exhibits an ultrahigh sensitivity to NO2 in dry air and the sensitivity is dependent on its thickness. A maximum response is observed for 4.8-nm-thick PNS, with a sensitivity up to 190% at 20 parts per billion (p.p.b.) at room temperature. First-principles calculations combined with the statistical thermodynamics modelling predict that the adsorption density is ∼10(15) cm(-2) for the 4.8-nm-thick PNS when exposed to 20 p.p.b. NO2 at 300 K. Our sensitivity modelling further suggests that the dependence of sensitivity on the PNS thickness is dictated by the band gap for thinner sheets (<10 nm) and by the effective thickness on gas adsorption for thicker sheets (>10 nm).

Solvent exfoliation of electronic-grade, two-dimensional black phosphorus.

Solution dispersions of two-dimensional (2D) black phosphorus (BP)--often referred to as phosphorene--are achieved by solvent exfoliation. These pristine, electronic-grade BP dispersions are produced with anhydrous organic solvents in a sealed-tip ultrasonication system, which circumvents BP degradation that would otherwise occur via solvated O2 or H2O. Among conventional solvents, N-methylpyrrolidone (NMP) is found to provide stable, highly concentrated (∼0.4 mg/mL) BP dispersions. Atomic force microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy show that the structure and chemistry of solvent-exfoliated BP nanosheets are comparable to mechanically exfoliated BP flakes. Additionally, residual NMP from the liquid-phase processing suppresses the rate of BP oxidation in ambient conditions. Solvent-exfoliated BP nanosheet field-effect transistors exhibit ambipolar behavior with current on/off ratios and mobilities up to ∼10(4) and ∼50 cm(2) V(-1) s(-1), respectively. Overall, this study shows that stable, highly concentrated, electronic-grade 2D BP dispersions can be realized by scalable solvent exfoliation, thereby presenting opportunities for large-area, high-performance BP device applications.

Effective passivation of exfoliated black phosphorus transistors against ambient degradation.

Unencapsulated, exfoliated black phosphorus (BP) flakes are found to chemically degrade upon exposure to ambient conditions. Atomic force microscopy, electrostatic force microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy are employed to characterize the structure and chemistry of the degradation process, suggesting that O2 saturated H2O irreversibly reacts with BP to form oxidized phosphorus species. This interpretation is further supported by the observation that BP degradation occurs more rapidly on hydrophobic octadecyltrichlorosilane self-assembled monolayers and on H-Si(111) versus hydrophilic SiO2. For unencapsulated BP field-effect transistors, the ambient degradation causes large increases in threshold voltage after 6 h in ambient, followed by a ∼ 10(3) decrease in FET current on/off ratio and mobility after 48 h. Atomic layer deposited AlOx overlayers effectively suppress ambient degradation, allowing encapsulated BP FETs to maintain high on/off ratios of ∼ 10(3) and mobilities of ∼ 100 cm(2) V(-1) s(-1) for over 2 weeks in ambient conditions. This work shows that the ambient degradation of BP can be managed effectively when the flakes are sufficiently passivated. In turn, our strategy for enhancing BP environmental stability will accelerate efforts to implement BP in electronic and optoelectronic applications.