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.     

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.     

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.     

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.     

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.     

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.     

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.     

石墨烯量子点:制备及检测乙醇-水溶液组分

采用改进的非完全脱水碳化方法制备石墨烯量子点.产物尺寸约6 nm,激发峰355 nm,发射峰458 nm.基于乙醇对石墨烯锯齿边质子化的抑制效应,发现可通过石墨烯量子点荧光光强检测乙醇-水混合溶液组分.在0.01-100%范围内,乙醇体积百分比与发射峰光强增量呈线性关系.本研究为制备石墨烯量子点提供参考,提出了简易的较宽范围的乙醇水溶液含量检测方法 .     

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.     

Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池数值模拟

银锌锡硒(Ag2ZnSnSe4)是一种禁带宽度为1.4 eV的n型半导体材料.本文提出一种由n型Ag2ZnSnSe4与石墨烯(Graphene)组成的Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池,并借助wxAMPS软件对电池的物理机理和性能影响因素进行模拟研究.模拟结果表明,高功函数的石墨烯与n型Ag2ZnSnSe4半导体接触时,Ag2ZnSnSe4吸收层的前端能带向上弯曲,在n型Ag2ZnSnSe4吸收层表面诱导形成p型Ag2ZnSnSe4反型层,p型Ag2ZnSnSe4和n型Ag2ZnSnSe4组成p-n同质结.模拟发现石墨烯和背接触的功函数会影响载流子的分离、输运和收集,严重影响器件性能,石墨烯功函数达到5.5 eV,背接触功函数不高于4.4 eV,都有利于提高器件性能.Ag2ZnSnSe4吸收层的掺杂浓度主要影响器件的短路电流,而Ag2ZnSnSe4吸收层的体内缺陷对器件整体性能产生影响.在石墨烯和背接触功函数分别为5.5和3.8 eV,Ag2ZnSnSe4吸收层的掺杂浓度和缺陷密度分别为1016和1014 cm–3时,Graphene/Ag2ZnSnSe4诱导p-n结薄膜太阳电池能够取得高达23.42%的效率.这些模拟结果为设计新型高效低成本太阳电池提供了思路和物理阐释.     

基于明胶制备碳量子点及其光学性能的研究

通过水热法采用热解明胶制备出有蓝色荧光的碳量子点,并通过单因素优化实验对制备碳量子点的温度、时间进行优化以选择出制备碳量子点的最佳条件,结果表明在水热反应温度为200 ℃,反应时间为6 h时制备的碳量子点的荧光性能最强。同时,利用透射电子显微镜(TEM)、傅里叶变换红外光谱(FTIR)、X射线光电子能谱(XPS)、Ｘ射线衍射(XRD)、紫外-可见吸收光谱(UV)及荧光光谱(PL)等手段对最佳条件下制备的碳量子点进行测试与表征,结果表明,该方法制备的碳量子点量子产率为39.4%,与不掺杂的碳量子点相比其量子产率相对较高,这可能是因为有N元素的存在使得量子产率有所提高;所制备的碳量子点不仅具有丰富的含氧官能团而且抗光漂白性能良好,形态主要是均匀分散的球形,没有明显的晶格条纹,这与相关文献报道的碳量子点的形态相一致,其在250～300 nm有较弱的吸收,但无明显的特征吸收峰,这可能是由于C=O基团的n-π*跃迁引起的;此外,还讨论了氙灯照射时间、pH、碳量子点浓度、不同类型溶剂及离子强度等因素对碳量子点荧光性能的影响,研究结果表明,氙灯照射时间及离子强度对碳量子点荧光性能几乎无影响,在过酸或过碱的条件下其荧光强度相对较弱,原因可能是在过酸或过碱的条件下发生质子化或非质子化的作用导致其荧光强度减弱;且碳量子点溶液随着其浓度的增加,荧光强度先增加后减小;而对于溶剂类型而言,其在极性溶剂中的荧光强度大于其在非极性溶剂中的荧光强度,说明该方法制备的碳量子点具有良好的水溶性。     

Doping-dependent evolution of low-energy excitations and quantum phase transitions within an effective model for high-T<Subscript>c</Subscript> copper oxides

In this paper a mean-field theory for the spin-liquid paramagnetic non-superconducting phase of the p- and n-type high-T_c cuprates is developed. This theory applied to the effective t-t'-t′′-J^* model with the ab initio calculated parameters and with the three-site correlated hoppings. The static spin-spin and kinematic correlation functions beyond Hubbard-I approximation are calculated self-consistently. The evolution of the Fermi surface and band dispersion is obtained for the wide range of doping concentrations x. For p-type systems the three different types of behavior are found and the transitions between these types are accompanied by the changes in the Fermi surface topology. Thus a quantum phase transitions take place at x = 0.15 and at x = 0.23.Due to the different Fermi surface topology we found for n-type cuprates only one quantum critical concentration, x = 0.2. The calculated doping dependence of the nodal Fermi velocity and the effective mass are in good agreement with the experimental data.     

Planar Dirac electrons in magnetic quantum dots

In this paper, we explore the size- and mass-dependent energy spectra and the electronic correlation of two- and three-electron graphene magnetic quantum dots. It is found that only the magnetic dots with large size can well confine the electrons. For large graphene magnetic dots with massless (ultra-relativity) electrons, the energy level structures of two Dirac electrons and even the ground state spin and angular momentum of three electrons are quite different from those of the usual semiconductor quantum dots. Also we reveal that such differences are not due to the magnetic confinement but originate from the character of the Coulomb interaction of two-component electronic wavefunctions in graphene. We reveal that the increase of the mass leads to both the crossover of the energy spectrum structures from the ultra-relativity to non-relativity ones and the increasing of the crystallization. The results are helpful for the understanding of the mass and size effects and may be useful in controlling the few-electron states in graphene-based nanodevices.     

Polarization spectroscopy of positive and negative trions in an InAs quantum dot

Using polarization-sensitive photoluminescence and photoluminescence excitation spectroscopy, we study single InAs/GaAs self-assembled quantum dots. The dots were embedded in an n-type, Schottky diode structure allowing for control of the charge state. We present here the exciton, singly charged exciton (positive and negative trions), and the twice negatively charged exciton. For non-resonant excitation below the wetting layer, we observed a large degree of polarization memory from the radiative recombination of both the positive and negative trions. In excitation spectra, through the p-shell, we have found several sharp resonances in the emission from the s-shell recombination of the dot in all charged states. Some of these excitation resonances exhibit strong coulomb shifts upon addition of charges into the quantum dot. One particular resonance of the negatively charged trion was found to exhibit a fine structure doublet under circular polarization. This observation is explained in terms of resonant absorption into the triplet states of the negative trion.     

Modulation of magnetic and electrical properties of bilayer graphene quantum dots using rotational stacking faults

Bilayer graphene quantum dots with rotational stacking faults(RSFs) having different rotational angles were studied.Using the first-principles calculation, we determined that these stacking faults could quantitatively modulate the magnetism and the distribution of spin and energy levels in the electronic structures of the dots.In addition, by examining the spatial distribution of unpaired spins and Bader charge analysis, we found that the main source of magnetic moment originated from the edge atoms of the quantum dots.Our research results can potentially provide a new path for producing all-carbon nanodevices with different electrical and magnetic properties.     

Multipole-based modal analysis of gate-defined quantum dots in graphene

A new numerical method based on the multipoles of the Dirac equation is presented for rigorous and fast analysis of electron scattering from gate-defined structures in graphene. The new method is used to study the strongly bound states and the weakly bound states of a circular quantum dot. The accuracy of the obtained results is then verified by the T-matrix method. Furthermore, we characterize the resonances of elliptical gate-defined quantum dots and compare these resonances with the strongly bound states of circular dots. The effects of coupling between two quantum dots are also investigated.     

Magnetic barriers and confinement of Dirac-Weyl quasiparticles in graphene

We discuss the properties of the two-dimensional massless Dirac-Weyl quasiparticles realized in graphene monolayers in the presence of inhomogeneous magnetic fields. We show that in contrast to electrostatic barriers, appropriate magnetic barriers are able to confine these quasiparticles. This allows for a novel way of designing mesoscopic structures (e.g., quantum dots, quantum point contacts) in graphene.