There have been rapidly increasing demands for flexible lighting apparatus, and micrometer-scale light-emitting diodes (LEDs) are regarded as one of the promising lighting sources for deformable device applications. Herein, we demonstrate a method of creating a deformable LED, based on remote heteroepitaxy of GaN microrod (MR) p-n junction arrays on c-Al2O3 wafer across graphene. The use of graphene allows the transfer of MR LED arrays onto a copper plate, and spatially separate MR arrays offer ideal device geometry suitable for deformable LED in various shapes without serious device performance degradation. Moreover, remote heteroepitaxy also allows the wafer to be reused, allowing reproducible production of MR LEDs using a single substrate without noticeable device degradation. The remote heteroepitaxial relation is determined by high-resolution scanning transmission electron microscopy, and the density functional theory simulations clarify how the remote heteroepitaxy is made possible through graphene.

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS flakes in our BP/WS van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 10, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

Hexagonal boron nitride (hBN) is an insulating two-dimensional (2D) material with a large bandgap. Although known for its interfacing with other 2D materials and structural similarities to graphene, the potential use of hBN in 2D electronics is limited by its insulating nature. Here, we report atomically sharp twin boundaries at AA′/AB stacking boundaries in chemical vapor deposition–synthesized few-layer hBN. We find that the twin boundary is composed of a 6′6′ configuration, showing conducting feature with a zero bandgap. Furthermore, the formation mechanism of the atomically sharp twin boundaries is suggested by an analogy with stacking combinations of AA′/AB based on the observations of extended Klein edges at the layer boundaries of AB-stacked hBN. The atomically sharp AA′/AB stacking boundary is promising as an ultimate 1D electron channel embedded in insulating pristine hBN. This study will provide insights into the fabrication of single-hBN electronic devices.

There have been a few studies of heterojunctions composed of two-dimensional transition-metal dichalcogenides (TMDs) and an oxide layer, but such studies of high-performance electric and optoelectronic devices are essential. Such heterojunctions with low-resistivity metal contacts are needed by the electronics industry to fabricate efficient diodes and photovoltaic devices. Here, a van der Waals heterojunction composed of p-type black phosphorus (p-BP) and n-type indium–gallium–zinc oxide (n-IGZO) films with low-resistivity metal contacts is reported, and it demonstrates high rectification. The low off-state leakage current in the thick IGZO film accounts for the high rectification ratio in a sharp interface of p-BP/n-IGZO devices. For electrostatic gate control, an ionic liquid is introduced to achieve a high rectification ratio of 9.1 × 104. The photovoltaic measurements of p-BP/n-IGZO show fast rise and decay times, significant open-circuit voltage and short-circuit current, and a high photoresponsivity of 418 mA/W with a substantial external quantum efficiency of 12.1%. The electric and optoelectronic characteristics of TMDs/oxide layer van der Waals heterojunctions are attractive for industrial applications in the near future.

In optoelectronic devices based on two-dimensional (2D) semiconductor heterojunctions, the efficient charge transport of photogenerated carriers across the interface is a critical factor to determine the device performances. Here, we report an unexplored approach to boost the optoelectronic device performances of the WSe2–MoS2 p–n heterojunctions via the monolithic-oxidation-induced doping and resultant modulation of the interface band alignment. In the proposed device, the atomically thin WO x layer, which is directly formed by layer-by-layer oxidation of WSe2, is used as a charge transport layer for promoting hole extraction. The use of the ultrathin oxide layer significantly enhanced the photoresponsivity of the WSe2–MoS2 p–n junction devices, and the power conversion efficiency increased from 0.7 to 5.0%, maintaining the response time. The enhanced characteristics can be understood by the formation of the low Schottky barrier and favorable interface band alignment, as confirmed by band alignment analyses and first-principle calculations. Our work suggests a new route to achieve interface contact engineering in the heterostructures toward realizing high-performance 2D optoelectronics.

We investigate the mechanism of dihydrogen adsorption onto Ca cation centers, which has been the significant focus of recent research for hydrogen storage. We particularly concentrate on reliability of commonly used density-functional theories, in comparison with correlated wave function theories. It is shown that, irrespective of the chosen exchange-correlation potentials, density-functional theories result in unphysical binding of H 2 molecules onto Ca 1þ system. This suggests that several previous publications could contain a serious overestimation of storage capacity at least in part of their results. DOI: 10.1103/PhysRevLett.103.216102 PACS numbers: 68.43.Bc, 84.60.Ve One of the greatest challenges of scientific communities worldwide is a pollution-free and renewable energy source. Hydrogen storage with high enough volumetric and gravimetric density is particularly important as an energy carrier for a mobile system [1,2]. It has been discussed that, in order for the storage and discharge to be cycled near room temperature, hydrogen adsorbents need to have a binding affinity with the hydrogen molecule of a few tens of kJ/mol [3,4]. Through a very particular chemistry between the open d shell of metal atoms and H 2 molecular orbitals, an optimal strength of hydrogen adsorption can be achieved [5,6]. In order to realize such a chemistry in the form of a practical hydrogen storage system, numerous previous theory articles investigated nanostructures with dispersed transition metal (TM) atoms [3,[7][8][9]. However, experimental trials to synthesize an open-TM-based hydrogen storage system have been unsuccessful for various reasons. The most prominent barrier is the strong tendency of aggregation of TM atoms which renders the suggested models of dispersed TMs rather hyphothetical [10]. As an alternative, numerous research groups are now focused toward the alkaline-earth metals (AEMs) which have less tendency of aggregation and are believed to have similar binding affinity with dihydrogen adsorbates as TMs [11][12][13][14][15]. In particular, the systems of dispersed Ca atoms have been suggested as possessing the most salient properties.In the series of computational searches for hydrogen storage materials, the density-functional theory (DFT) has been used most widely, mainly because of its practicality [16]. However, common implementations of DFT involve approximations in the exchange-correlation potential, and thus the accuracy of DFTs could not be perfectly trusted. In this regard, it is very pertinent to investigate the capability of common forms of DFT for hydrogen adsorption onto AEMs. We show that DFTs deviate significantly from the correlated wave function theories in the description of the dihydrogen adsorption onto the Ca 1þ system. We discuss that the valence configuration of the Ca cation can be sharply switched between 4s and 3d upon hydrogen adsorption [17]. This sort of interaction is not widely noticed, and the reliability of DFTs for that is intriguing in the context of hydrogen storage as wel...

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We report an effective and reliable method to increase the work function of graphene to as high as 5.50 eV by applying a self-assembled monolayer on its surface. The work function of pristine graphene (4.56 eV) was increased by approximately þ0.94 eV following trichlorosilane (HDF-S) self-assembly. This increase in the work function was confirmed by ab initio calculations. HDF-S self-assembled graphene exhibited no significant changes in structural, optical, or electrical characteristics compared with pristine graphene. In addition, we verified that the modified work function of HDF-S self-assembled graphene was not affected by the underlying substrates. V C 2014 AIP Publishing LLC. [http://dx.