CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity.

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of n-alkyl chains anchored with acetylene groups (CnA; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (I-V) characteristics of CnA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance GAg < GAu < GPt is found to follow the same ordering as the metal work function ΦAu < ΦAu < ΦPt, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CnT) to which we refer in view of some similarities. Furthermore, unlike in CnT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CnA an additional MO shift, whose independence of size (n) rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CnA and CnT junctions different. What ultimately makes CnA unique at the microscopic level is a salient difference never reported previously, namely, that CnA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CnA from the CnT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.

Graphene-In2Se3 van der Waals Heterojunction Neuristor for Optical In-Memory Bimodal Operation.

Functional diversification at the single-device level has become essential for emerging optical neural network (ONN) development. Stable ferroelectricity harnessed with strong light sensitivity in α-In2Se3 holds great potential for developing ultrathin neuromorphic devices. Herein, we demonstrated an all-2D van der Waals heterostructure-based programmable synaptic field effect transistor (FET) utilizing a ferroelectric α-In2Se3 nanosheet and monolayer graphene. The devices exhibited reconfigurable, multilevel nonvolatile memory (NVM) states, which can be successively modulated by multiple dual-mode (optical and electrical) stimuli and thereby used to realize energy-efficient, heterosynaptic functionalities in a biorealistic fashion. Furthermore, under light illumination, the prototypical device can toggle between volatile (photodetector) and nonvolatile optical random-access memory (ORAM) logic operation, depending upon the ferroelectric-dipole induced band adjustment. Finally, plasticity modulation from short-term to prominent long-term characteristics over a wide dynamic range was demonstrated. The inherent operation mechanism owing to the switchable polarization-induced electronic band alignment and bidirectional barrier height modulation at the heterointerface was revealed by conjugated electronic transport and Kelvin-probe force microscopy (KPFM) measurements. Overall, robust (opto)electronic weight controllability for integrated in-sensor and in-memory logic processors and multibit ORAM systems was readily accomplished by the synergistic ferrophotonic heterostructure properties. Our presented results facilitate the technological implementation of versatile all-2D heterosynapses for next-generation perception, optoelectronic logic systems, and Internet-of-Things (IoT) entities.

Valley-addressable monolayer lasing through spin-controlled Berry phase photonic cavities.

The spin-valley coupling between circularly polarized light and valley excitons in transition metal dichalcogenides provides the opportunity to generate and manipulate spin information by exploiting the valley degree of freedom. Here, we demonstrate a room-temperature valley-addressable tungsten disulfide monolayer laser in which the spin of lasing is controlled by the spin of pump without magnetic fields. This effect was achieved by integrating a tungsten disulfide monolayer into a photonic cavity that supports two orthogonal spin modes with high quality factors. The spin-pumped lasing effectively broke the population symmetry of valley excitons, resulting in highly coherent emission with valley-switchable radiation modes due to distinct laser thresholds. Our scheme provides a nanophotonic platform to develop versatile coherent spin-light sources operating at room temperature by actively manipulating spin-valley coupling in light-matter interactions.

Spin-valley Rashba monolayer laser.

Direct-bandgap transition metal dichalcogenide monolayers are appealing candidates to construct atomic-scale spin-optical light sources owing to their valley-contrasting optical selection rules. Here we report on a spin-optical monolayer laser by incorporating a WS2 monolayer into a heterostructure microcavity supporting high-Q photonic spin-valley resonances. Inspired by the creation of valley pseudo-spins in monolayers, the spin-valley modes are generated from a photonic Rashba-type spin splitting of a bound state in the continuum, which gives rise to opposite spin-polarized ±K valleys due to emergent photonic spin-orbit interaction under inversion symmetry breaking. The Rashba monolayer laser shows intrinsic spin polarizations, high spatial and temporal coherence, and inherent symmetry-enabled robustness features, enabling valley coherence in the WS2 monolayer upon arbitrary pump polarizations at room temperature. Our monolayer-integrated spin-valley microcavities open avenues for further classical and non-classical coherent spin-optical light sources exploring both electron and photon spins.

Edge-Based Two-Dimensional α-In2Se3-MoS2 Ferroelectric Field Effect Device.

Heterostructures based on two-dimensional materials offer the possibility to achieve synergistic functionalities, which otherwise remain secluded by their individual counterparts. Herein, ferroelectric polarization switching in α-In2Se3 has been utilized to engineer multilevel nonvolatile conduction states in a partially overlapping α-In2Se3-MoS2-based ferroelectric semiconducting field effect device. In particular, we demonstrate how the intercoupled ferroelectric nature of α-In2Se3 allows to nonvolatilely switch between n-i and n-i-n type junction configurations based on a novel edge state actuation mechanism, paving the way for subnanometric scale nonvolatile device miniaturization. Furthermore, the induced asymmetric polarization enables enhanced photogenerated carriers' separation, resulting in an extremely high photoresponse of ∼1275 A/W in the visible range and strong nonvolatile modulation of the bright A- and B- excitonic emission channels in the overlaying MoS2 monolayer. Our results show significant potential to harness the switchable polarization in partially overlapping α-In2Se3-MoS2 based FeFETs to engineer multimodal, nonvolatile nanoscale electronic and optoelectronic devices.

Edge State Quantum Interference in Twisted Graphitic Interfaces.

Zigzag edges in graphitic systems exhibit localized electronic states that drastically affect their properties. Here, room-temperature charge transport experiments across a single graphitic interface are reported, in which the interlayer current is confined to the contact edges. It is shown that the current exhibits pronounced oscillations of up to ≈40 µA with a dominant period of ≈5 Å with respect to lateral displacement that do not directly correspond to typical graphene lattice spacing. The origin of these features is computationally rationalized as quantum mechanical interference of localized edge states showing significant amplitude and interlayer coupling variations as a function of the interface stacking configuration. Such interference effects may therefore dominate the transport properties of low-dimensional graphitic interfaces.

Bright excitonic multiplexing mediated by dark exciton transition in two-dimensional TMDCs at room temperature.

2D-semiconductors with strong light-matter interaction are attractive materials for integrated and tunable optical devices. Here, we demonstrate room-temperature wavelength multiplexing of the two-primary bright excitonic channels (Ab-, Bb-) in monolayer transition metal dichalcogenides (TMDs) arising from a dark exciton mediated transition. We present how tuning dark excitons via an out-of-plane electric field cedes the system equilibrium from one excitonic channel to the other, encoding the field polarization into wavelength information. In addition, we demonstrate how such exciton multiplexing is dictated by thermal-scattering by performing temperature dependent photoluminescence measurements. Finally, we demonstrate experimentally and theoretically how excitonic mixing can explain preferable decay through dark states in MoX2 in comparison with WX2 monolayers. Such field polarization-based manipulation of excitonic transitions can pave the way for novel photonic device architectures.

In Situ Observation of Low-Power Nano-Synaptic Response in Graphene Oxide Using Conductive Atomic Force Microscopy.

Multiple studies have reported the observation of electro-synaptic response in different metal/insulator/metal devices. However, most of them analyzed large (>1 µm2 ) devices that do not meet the integration density required by industry (1010  devices/mm2 ). Some studies emploied a scanning tunneling microscope (STM) to explore nano-synaptic response in different materials, but in this setup there is a nanogap between the insulator and one of the metallic electrodes (i.e., the STM tip), not present in real devices. Here, it is demonstrated how to use conductive atomic force microscopy to explore the presence and quality of nano-synaptic response in confined areas <50 nm2 . Graphene oxide (GO) is selected due to its easy fabrication. Metal/GO/metal nano-synapses exhibit potentiation and paired pulse facilitation with low write current levels <1 µA (i.e., power consumption ≈3 µW), controllable excitatory post-synaptic currents, and long-term potentiation and depression. The results provide a new method to explore nano-synaptic plasticity at the nanoscale, and point to GO as an important candidate for the fabrication of ultrasmall (<50 nm2 ) electronic synapses fulfilling the integration density requirements of neuromorphic systems.

Modulating the Optoelectronic Properties of MoS2 by Highly Oriented Dipole-Generating Monolayers.

The noncovalent functionalization of two-dimensional materials (2DMs) with bespoke organic molecules is of central importance for future nanoscale electronic devices. Of particular interest is the incorporation of molecular functionalities that can modulate the physicochemical properties of the 2DMs via noninvasive external stimuli. In this study, we present the reversible modulation of the photoluminescence, spectroscopic properties (Raman), and charge transport characteristics of molybdenum disulfide (MoS2)-based devices via photoisomerization of a self-assembled monolayer of azobenzene-modified triazatriangulene molecules. The observed (opto)electronic modulations are explained by the n-type doping of the MoS2 lattice induced by the photoisomerization of the highly ordered azobenzene monolayer. This novel behavior could have profound effects on future composite 2DM-based (opto)electronics.

Dipole-Induced Raman Enhancement Using Noncovalent Azobenzene-Functionalized Self-Assembled Monolayers on Graphene Terraces.

Graphene is a promising material in the field of interface science, especially for noncovalent functionalization, sensing, and for applications in catalysis and nanoelectronics. The noncovalent self-assembly of aromatic molecules on graphene promotes electronic coupling through π-π interactions that allows for quenching of the fluorescence of adsorbent molecules and the enhancement of their Raman spectra via graphene-enhanced Raman spectroscopy (GERS). Although recent work has explored the Raman enhancement on mono- and bilayer graphene, the layer dependence of both electronic phenomena (i.e., fluorescence quenching and Raman enhancement) has largely remained underexplored. Similarly, the effect of near-surface molecular dipoles on GERS has sparsely been examined. In this work, we employ self-assembled monolayers of azobenzene-decorated triazatriangulene molecules (AzoTATA) on graphene terraces to examine the effect of switchable molecular dipoles on the GERS effect, which occurs as a function of azobenzene photoisomerization. Furthermore, using empirical and computational methods, we present a systematic study for deriving the mechanism of GERS enhancement and fluorescence quenching on graphene terraces.

Maskless Device Fabrication and Laser-Induced Doping in MoS2 Field Effect Transistors Using a Thermally Activated Cyclic Polyphthalaldehyde Resist.

We present a novel maskless device fabrication technique for rapid prototyping of two-dimensional (2D)-based electronic materials. The technique is based on a thermally activated and self-developed cyclic polyphthalaldehyde (c-PPA) resist using a commercial Raman system and 532 nm laser illumination. Following the successful customization of electrodes to form field effect transistors based on MoS2 monolayers, the laser-induced electronic doping of areas beneath the metal contacts that were exposed during lithography was investigated using both surface potential mapping and device characterization. An effective change in the doping level was introduced depending on the laser intensity, i.e., low laser powers resulted in p-doping, while high laser powers resulted in n-doping. Fabricated devices present a low contact resistance down to 10 kΩ·µm at a back-gate voltage of VG = 80 V, which is attributed to the laser-induced n-type doping at the metal contact regions.

Scalable Integration of Coplanar Heterojunction Monolithic Devices on Two-Dimensional In2Se3.

The formation of lateral heterojunction arrays within two-dimensional (2D) crystals is an essential step to realize high-density, ultrathin electro-optical integrated circuits, although the assembling of such structures remains elusive. Here we demonstrated a rapid, scalable, and site-specific integration of lateral 2D heterojunction arrays using few-layer indium selenide (In2Se3). We use a scanning laser probe to locally convert In2Se3 into In2O3, which shows a significant increase in carrier mobility and transforms the metal-semiconductor junctions from Schottky to ohmic type. In addition, a lateral p-n heterojunction diode within a single nanosheet is demonstrated and utilized for photosensing applications. The presented method enables high-yield, site-specific formation of lateral 2D In2Se3-In2O3-based hybrid heterojunctions for realizing nanoscale devices with multiple advanced functionalities.

The scaling laws of edge vs. bulk interlayer conduction in mesoscale twisted graphitic interfaces.

The unusual electronic properties of edges in graphene-based systems originate from the pseudospinorial character of their electronic wavefunctions associated with their non-trivial topological structure. This is manifested by the appearance of pronounced zero-energy electronic states localized at the material zigzag edges that are expected to have a significant contribution to the interlayer transport in such systems. In this work, we utilize a unique experimental setup and electronic transport calculations to quantitatively distinguish between edge and bulk transport, showing that their relative contribution strongly depends on the angular stacking configuration and interlayer potential. Furthermore, we find that, despite of the strong localization of edge state around the circumference of the contact, edge transport in incommensurate interfaces can dominate up to contact diameters of the order of 2 µm, even in the presence of edge disorder. The intricate interplay between edge and bulk transport contributions revealed in the present study may have profound consequences on practical applications of nanoscale twisted graphene-based electronics.

Photonic Rashba effect from quantum emitters mediated by a Berry-phase defective photonic crystal.

Heterostructures combining a thin layer of quantum emitters and planar nanostructures enable custom-tailored photoluminescence in an integrated fashion. Here, we demonstrate a photonic Rashba effect from valley excitons in a WSe2 monolayer, which is incorporated into a photonic crystal slab with geometric phase defects, that is, into a Berry-phase defective photonic crystal. This phenomenon of spin-split dispersion in momentum space arises from a coherent geometric phase pickup assisted by the Berry-phase defect mode. The valley excitons effectively interact with the defects for site-controlled excitation, photoluminescence enhancement and spin-dependent manipulation. Specifically, the spin-dependent branches of photoluminescence in momentum space originate from valley excitons with opposite helicities and evidence the valley separation at room temperature. To further demonstrate the versatility of the Berry-phase defective photonic crystals, we use this concept to separate opposite spin states of quantum dot emission. This spin-enabled manipulation of quantum emitters may enable highly efficient metasurfaces for customized planar sources with spin-polarized directional emission.

Formation of Highly Ordered Self-Assembled Monolayers on Two-Dimensional Materials via Noncovalent Functionalization.

Functionalized two-dimensional materials (2DMs) are attracting much attention due to their promising applications in nanoscale devices. Producing continuous and homogeneous surface assemblies with a high degree of order has been challenging. In this work, we demonstrate that by noncovalently self-assembling molecular platforms on 2DMs, high-quality and highly ordered monolayers can be generated. The high degree of order and uniformity of the self-assembled monolayer layers were confirmed by a variety of analytic techniques including time-of-flight secondary ion mass spectrometry, scanning tunnelling microscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. Furthermore, by selectively enhancing the molecular vibrations of the molecular platform, via a combination of graphene-enhanced Raman spectroscopy (GERS) and surface-enhanced Raman spectroscopy (SERS), we were able to determine the orientation of self-assembled molecular platforms with respect to the surface normal. The selective enhancement of the vibrational modes occurs by taking advantage of the distance dependence of the Raman enhancement either by the graphene surface (GERS) or the silver nanoparticules (SERS) that are located on top of the self-assembled monolayer.

The dielectric constant of a bilayer graphene interface.

The interlayer relative dielectric constant, ε r, of 2-dimensional (2D) materials in general and graphitic materials in particular is one of their most important physical properties, especially for electronic applications. In this work, we study the electromechanical actuation of nano-scale graphitic contacts. We find that beside the adhesive forces there are capacitive forces that scale parabolically with the potential drop across the sheared interface. We use this phenomena to measure the intrinsic dielectric constant of the bilayer graphene interface i.e. ε r = 6 ± 2, which is in perfect agreement with recent theoretical predictions for multi-layer graphene structures. Our method can be generally used to extract the dielectric properties of 2D materials systems and interfaces and our results pave the way for utilizing graphitic and other 2D materials in electromechanical based applications.

Coherent commensurate electronic states at the interface between misoriented graphene layers.

Graphene and layered materials in general exhibit rich physics and application potential owing to their exceptional electronic properties, which arise from the intricate π-orbital coupling and the symmetry breaking in twisted bilayer systems. Here, we report room-temperature experiments to study electrical transport across a bilayer graphene interface with a well-defined rotation angle between the layers that is controllable in situ. This twisted interface is artificially created in mesoscopic pillars made of highly oriented pyrolytic graphite by mechanical actuation. The overall measured angular dependence of the conductivity is consistent with a phonon-assisted transport mechanism that preserves the electron momentum of conduction electrons passing the interface. The most intriguing observations are sharp conductivity peaks at interlayer rotation angles of 21.8° and 38.2°. These angles correspond to a commensurate crystalline superstructure leading to a coherent two-dimensional (2D) electronic interface state. Such states, predicted by theory, form the basis for a new class of 2D weakly coupled bilayer systems with hitherto unexplored properties and applications.

Surface science. Adhesion and friction in mesoscopic graphite contacts.

The weak interlayer binding in two-dimensional layered materials such as graphite gives rise to poorly understood low-friction characteristics. Accurate measurements of the adhesion forces governing the overall mechanical stability have also remained elusive. We report on the direct mechanical measurement of line tension and friction forces acting in sheared mesoscale graphite structures. We show that the friction is fundamentally stochastic in nature and is attributable to the interaction between the incommensurate interface lattices. We also measured an adhesion energy of 0.227 ± 0.005 joules per square meter, in excellent agreement with theoretical models. In addition, bistable all-mechanical memory cell structures and rotational bearings have been realized by exploiting position locking, which is provided solely by the adhesion energy.

Measurement of active dopant distribution and diffusion in individual silicon nanowires.

We have measured the radial distribution and diffusion of active dopant atoms in individual silicon nanowires grown by the vapor-liquid-solid (VLS) method. Our method is based on successive surface etching of a portion of a contacted nanowire, followed by measurement of the potential difference between the etched and unetched areas using Kelvin probe force microscopy (KPFM). The radial dopant distribution is obtained by fitting the measured potentials with a three-dimensional solution of Poisson equation. We find that the radial active dopant distribution decreases by almost 2 orders of magnitude from the wire surface to its core even when there is no indication for tapering. In addition, the dopant profile is consistent with a very large diffusion coefficient of D approximately 1 x 10(-19) m(2) s(-1). This implies that phosphorus (P) diffusion during the VLS growth is remarkably high and subsequent thermal annealing must be used when a homogeneous dopant distribution is required.

Optical waveguide enhanced photovoltaics.

Enhanced light to electric power conversion efficiency of photovoltaic cells with a low absorbance was achieved using waveguide integration. We present a proof of concept using a very thin dye-sensitized solar cell which absorbed only a small fraction of the light at normal incidence. The glass substrate in conjunction with the solar cells reflecting back contact formed a planar waveguide, which lead to more than four times higher conversion efficiency compared to conventional illumination at normal incidence. This illumination concept leads to a new type of multi-junction PV systems based on enforced spectral splitting along the waveguide.