Optical properties of modulation-doped InGaAs vertically coupled quantum dots

Optical spectroscopy including photoluminescence, electroluminescence, photocurrent, and differential absorption, have been investigated for the triple-layer InGaAs vertically coupled quantum dots (VCQDs) by adding modulation doping (MD) in the 5 nm GaAs spacer layers. In addition to the QDs fundamental and excited transitions, a coupled-state transition is observed for the VCQDs. For the VCQDs of p-type MD, the optical transitions at ground state and coupled state are enhanced by the improvement of hole capture for the valence subbands. For the VCQDs of n-type MD, the main absorption change occurs at the coupled state, consistent with the dominant emission peak observed in EL spectra.     

Tuning the Schottky barrier in the arsenene/graphene van der Waals heterostructures by electric field

Using density functional theory calculations, we investigate the electronic properties of arsenene/graphene van der Waals (vdW) heterostructures by applying external electric field perpendicular to the layers. It is demonstrated that weak vdW interactions dominate between arsenene and graphene with their intrinsic electronic properties preserved. We find that an n-type Schottky contact is formed at the arsenene/graphene interface with a Schottky barrier of 0.54 eV. Moreover, the vertical electric field can not only control the Schottky barrier height but also the Schottky contacts (n-type and p-type) and Ohmic contacts (n-type) at the interface. Tunable p-type doping in graphene is achieved under the negative electric field because electrons can transfer from the Dirac point of graphene to the conduction band of arsenene. The present study would open a new avenue for application of ultrathin arsenene/graphene heterostructures in future nano- and optoelectronics.     

Characteristics of field-effect transistors based on undoped and B- and N-doped few-layer graphenes

Field-effect transistor characteristics of few-layer graphenes prepared by several methods have been investigated in comparison with those of single-layer graphene prepared by the in situ reduction of single-layer graphene oxide. Ambipolar features have been observed with single-layer graphene and n-type behaviour with all the few-layer graphenes, the best characteristics being found with the graphene possessing 2–3 layers prepared by arc-discharge of graphite in hydrogen. FETs based on boron and nitrogen doped graphene show n-type and p-type behaviour respectively.     

边界对石墨烯量子点非线性光学性质的影响

石墨烯作为一种新型非线性光学材料,在光子学领域具有重要的应用前景,引起研究人员的极大兴趣.本文运用量子化学计算方法研究了边界引入碳碳双键(C=C)和掺杂环硼氮烷(B₃N₃)环对石墨烯量子点非线性光学性质和紫外-可见吸收光谱的影响.研究发现,扶手椅边界上引入C=C双键后,六角形石墨烯量子点分子结构对称性降低,电荷分布对称性发生破缺,导致分子二阶非线性光学活性增强.石墨烯量子点在从扶手椅型边界向锯齿型边界过渡的过程中,随着边界C=C双键数目的增加,六角形石墨烯量子点和B3N3掺杂六角形石墨烯量子点的极化率和第二超极化率分别呈线性增加.此外,边界对石墨烯量子点的吸收光谱也有重要影响.无论是石墨烯量子点还是B₃N₃掺杂石墨烯量子点,扶手椅型边界上引入C=C双键导致最高占据分子轨道能级升高,最低未占分子轨道能级的降低,前线分子轨道能级差减小,因而最大吸收波长发生了红移.中心掺杂B₃N₃环后会增大石墨烯量子点的分子前线轨道能级差,导致B3N3掺杂后的石墨烯量子点紫外-可见吸收光谱发生蓝移.本文研究为边界修饰调控石墨烯量子点非线性光学响应提供了一定的理论指导.     

Transport in graphene nanostructures

Graphene nanostructures are promising candidates for future nanoelectronics and solid-state quantum information technology. In this review we provide an overview of a number of electron transport experiments on etched graphene nanostructures. We briefly revisit the electronic properties and the transport characteristics of bulk, i.e., two-dimensional graphene. The fabrication techniques for making graphene nanostructures such as nanoribbons, single electron transistors and quantum dots, mainly based on a dry etching ??paper-cutting?? technique are discussed in detail. The limitations of the current fabrication technology are discussed when we outline the quantum transport properties of the nanostructured devices. In particular we focus here on transport through graphene nanoribbons and constrictions, single electron transistors as well as on graphene quantum dots including double quantum dots. These quasi-one-dimensional (nanoribbons) and quasi-zero-dimensional (quantum dots) graphene nanostructures show a clear route of how to overcome the gapless nature of graphene allowing the confinement of individual carriers and their control by lateral graphene gates and charge detectors. In particular, we emphasize that graphene quantum dots and double quantum dots are very promising systems for spin-based solid state quantum computation, since they are believed to have exceptionally long spin coherence times due to weak spin-orbit coupling and weak hyperfine interaction in graphene.     

Modulation of Schottky barrier in XSi2N4/graphene (X=Mo and W) heterojunctions by biaxial strain

Reducing the Schottky barrier height (SBH) and even achieving the transition from Schottky contacts to Ohmic contacts are key challenges of achieving high energy efficiency and high-performance power devices. In this paper, the modulation effects of biaxial strain on the electronic properties and Schottky barrier of MoSi₂N₄ (MSN)/graphene and WSi₂N₄ (WSN)/graphene heterojunctions are examined by using first principles calculations. After the construction of heterojunctions, the electronic structures of MSN, WSN, and graphene are well preserved. Herein, we show that by applying suitable external strain to a heterojunction stacked by MSN or WSN — an emerging two-dimensional (2D) semiconductor family with excellent mechanical properties — and graphene, the heterojunction can be transformed from Schottky p-type contacts into n-type contacts, even highly efficient Ohmic contacts, making it of critical importance to unleash the tremendous potentials of graphene-based van der Waals (vdW) heterojunctions. Not only are these findings invaluable for designing high-performance graphene-based electronic devices, but also they provide an effective route to realizing dynamic switching either between n-type and p-type Schottky contacts, or between Schottky contacts and Ohmic contacts.     

Tuning the photoluminescence of graphene quantum dots by co-doping of nitrogen and sulfur

Nitrogen and sulfur co-doped graphene quantum dots (NS-GQDs) were successfully synthesized using a facile, inexpensive, and environmentally friendly hydrothermal reaction of aqueous ammonia, S powder, and graphene quantum dots. The NS-GQDs with oxygen-rich functional groups have a high quantum yield of 41% and a diameter of 1–5 nm. The photoluminescence (PL) properties of the nitrogen-doped graphene quantum dots (N-GQDs) and NS-GQD were investigated. The results showed that the PL emission of the NS-GQD exhibits a clear blue shift of 54 nm compared to that of the N-GQDs.     

Formation of graphene quantum dots by “Planting” hydrogen atoms at a graphene nanoribbon

Different technological approaches for creating graphene quantum dots by the adsorption of hydrogen atoms are considered. The adsorption can occur both at convex portions of a distorted graphene nanoribbon and in the structure formed by two distorted graphene nanoribbon rows superimposed on each other at the places free from the ribbon crossings. It is shown that settlement of hydrogen atoms at convex portions of the nanoribbons is energetically favorable. This gives rise to the creation of insulating graphane (CH) nanodomains separating the conducting regions. As a result, a graphene quantum dot appears. The variation of the electron spectra of graphene quantum dots with the length of these graphane regions is discussed.     

P–n junction characteristics of graphene oxide and reduced graphene oxide on n-type Si(111)

The semiconductor behavior of graphene oxide (GO) and reduced graphene oxide (RGO) synthesized by the Hummers method on n-type Si(111) were investigated. Graphene oxide is a product of the oxidation of graphite, during which numerous oxygen functional groups bond to the carbon plane during oxidation. RGO was prepared by adding excess hydrazine to the GO showing p-type semiconductor material behavior. In the C–O bond, the O atom tends to pull electrons from the C atom, leaving a hole in the carbon network. This results in p-type semiconductor behavior of GO, with the carrier concentration dependent upon the degree of oxidation. The RGO was obtained by removing most of the oxygen-containing functionalities from the GO using hydrazine. However, oxygen remaining on the carbon plane caused the RGO to exhibit p-type behavior. The I–V characteristics of GO and RGO deposited on n-type Si(111) forming p–n junctions exhibited different turn-on voltages and slope values.