Unveiling metal-cage hybrid states in a single endohedral metallofullerene

The local structural and electronic properties of individual metallofullerenes are studied using scanning tunneling microscopy, scanning tunneling spectroscopy, and theoretical simulations. The energy-resolved metal-cage hybrid states of a single endohedral metallofullerene Dy@C82 isomer I have been spatially mapped, supporting a complex picture consisting of the orbital hybridization and charge transfer for the interaction between the cage and the metal atom. The relative position of the encapsulated Dy atom inside the cage and the molecular orientation on the surface have been inferred by comparing the experimental results with theoretical simulations. The combined technique provides promising applications in the fields of in situ characterization and diagnostics of metallofullerene-based nanodevices.     

Single quantum dots as scanning tunneling microscope tips

We report the use of single quantum dot structures as tips on a scanning tunneling microscope (STM). A single quantum dot structure with a diameter of less than 200 nm and a height of 2 μm was fabricated by reactive ion etching. This dot was placed on a 40 μm-high mesa and mounted on the tip of a STM. The topography of large structures such as quantum wires or gold test substrates is clearly resolved with such a tip. To check the transport properties of the tip, quantum dot arrays were fabricated on resonant tunneling double barrier structures using the same process parameters. Conventional tunneling spectroscopy clearly resolved the 0D states in our samples. Using a metal substrate as second electrode such STM tips can be used to perform high resolution energy spectroscopy on single dots and free standing wire structures.     

STM images of a large organic molecule adsorbed on a bare metal substrate or on a thin insulating layer: Visualization of HOMO and LUMO

An isomer of the methylterrylene molecule was adsorbed both on Cu(111) and on a NaCl bilayer deposited on Cu(111) and imaged by ultra high vacuum scanning tunneling microscopy at low temperature (5 K). On the bare metal surface, the STM images do not reveal any intramolecular resolution and do not depend on the applied tunnel bias. On the contrary, the images acquired at specific bias voltages for the molecule on the salt layer show a striking similarity with the spatial distribution of the electronic probability density in the highest occupied molecular orbital (HOMO) and in the lowest unoccupied molecular orbital (LUMO) of free methylterrylene. They are well reproduced by elastic scattering quantum chemistry calculations. These data provide a direct view of the hyperconjugative interaction between the methyl group and the frontier orbitals of terrylene.     

Competition between quantum spin tunneling and Kondo effect

Quantum spin tunneling and Kondo effect are two very different quantum phenomena that produce the same effect on quantized spins, namely, the quenching of their magnetization. However, the nature of this quenching is very different so that quantum spin tunneling and Kondo effect compete with each other. Importantly, both quantum spin tunneling and Kondo effect produce very characteristic features in the spectral function that can be measured by means of single spin scanning tunneling spectroscopy and allows to probe the crossover from one regime to the other. We model this crossover, and the resulting changes in transport, using a non-perturbative treatment of a generalized Anderson model including magnetic anisotropy that leads to quantum spin tunneling. We predict that, at zero magnetic field, integer spins can feature a split-Kondo peak driven by quantum spin tunneling.     

Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise

We demonstrate that transport spectroscopy of single molecular magnets shows signatures of quantum tunneling at low temperatures. We find current and noise oscillations as a function of bias voltage due to a weak violation of spin-selection rules by quantum tunneling processes. The interplay with Boltzmann suppression factors leads to fake resonances with temperature-dependent position which do not correspond to any charge excitation energy. Furthermore, we find that quantum tunneling can completely suppress transport if the transverse anisotropy has a high symmetry.     

Direct observation of the quantum tunneling of single hydrogen atoms with a scanning tunneling microscope

Single hydrogen atoms were imaged on the Cu(001) surface by scanning tunneling microscopy (STM). The vibrations of individual H and D atoms against the surface were excited and detected by inelastic electron tunneling spectroscopy (STM-IETS). Variable temperature measurements of H atom diffusion showed a transition from thermally activated diffusion to quantum tunneling at 60 K. Regimes of phonon-assisted and electron-limited quantum tunneling were observed. The thermal diffusion rate of D atoms varied over 7 orders of magnitude between 80 and 50 K with no transition to quantum tunneling down to a thermal hopping rate of 4x10(-7) s(-1).     

Spin-polarized tunneling in a ferromagnetic graphene junction: Interplay between the exchange interaction and the orbital effect of the magnetic field

The influence of magnetic vector potential barrier (MVPB) on the spin-polarized transport of massless Dirac particles in ferromagnetic graphene is studied theoretically. The phenomenon of Klein tunneling of relativistic particles across a rectangular potential barrier prevents any of the massless fermions from being confined but they can be electrically confined by quantum dots with integrable dynamics (Bardarson et al., 2009) [36]. Utilization of only the in-plane exchange splitting in the ferromagnetic graphene cannot produce 100% spin polarization. This tunneling can be confined using the magnetic vector potential barrier, which leads to high degree of spin polarization. By combining the orbital effect and the Zeeman interaction in graphene junction, it is found that the junction mimics behavior of half-metallic tunneling junction, in which it acts as a metal to particles of one spin orientation but as an insulator or a semiconductor to those of the opposite orientation. The idea of the half-metallic tunneling junction can provide a source of ∼100% spin-polarized current, which is potentially very useful. Adjustment of the position of the Fermi level in ferromagnetic layer by placing a gate voltage on top of the ferromagnetic layer shows that reverse of the orientation of the completely spin-polarized current passing through the junction is controlled by adjusting the gate voltage. These interesting characteristics should lead to a practical gate voltage controlled spin filtering and spin-polarized switching devices as a perfect spin-polarized electron source for graphene-based spintronics.     

Scanned probe imaging of nanoscale conducting channels in Pt/alkanoic acid monolayer/Ti devices

The mechanisms responsible for switching in metal/molecule/metal systems are subjects of intense research. We report here a scanned probe technique that maps the conductance of a planar molecular junction (Pt/stearic acid monolayer/Ti) under mechanical perturbation by using an atomic force microscope (AFM) to apply a localized force to a molecular junction while measuring the junction conductance. Such mechano-conductance maps reveal that transport through the molecular device is dominated by nanoscale conducting channels, which emerged or disappeared when the junction is switched into higher or lower conductance states. A quantitative model that combines quantum tunneling with the growth of nano-asperities effectively describes the experimental conductance data across a wide range of device conductance. PACS 68.37.Ps; 73.23.-b; 73.63.-b; 81.07.Pr; 85.65.+h     

Voltage-controlled storage and retrieval of an infrared-light pulse in a quantum-dot molecule

Dynamic storage and retrieval of a weak infrared (IR)-light pulse are investigated theoretically with feasible parameters in an asymmetric double quantum dot system, a quantum dot molecule (QDM). It is shown that, with a voltage-controlled tunneling, we are able to store and retrieve the IR signal pulse in this three-subband QDM medium by slowly switching off and on the tunneling. The scheme proposed may open up the electrical controllability of quantum optical information storage and retrieval, which is expected to be useful in quantum information science in an asymmetric double quantum dot controlled by voltage.     

Effect of carbon nanotubes chirality on the E–C photo-isomerization switching behavior in moelcular device

Applying nonequilibrium Green's function formalism in combination with the first-principles density functional theory, we investigate the electronic transport properties of optical molecular switch based on the fulgide molecule with two different single-walled carbon nanotube (SWCNT) electrodes. The molecule that comprises the switch can convert between E isomer and C isomer by ultraviolet or visible irradiation. Theoretical results show that these two isomers exhibit very different conductance properties both in armchair and zigzag junction, which can realize the on and off states of the molecular switch. Meantime, the chirality of the SWCNT electrodes strongly affects the switching characteristics of the molecular junctions, which is useful for the design of functional molecular devices.     

Recent progresses of quantum confinement in graphene quantum dots

Graphene quantum dots (GQDs) not only have potential applications on spin qubit, but also serve as essential platforms to study the fundamental properties of Dirac fermions, such as Klein tunneling and Berry phase. By now, the study of quantum confinement in GQDs still attract much attention in condensed matter physics. In this article, we review the experimental progresses on quantum confinement in GQDs mainly by using scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Here, the GQDs are divided into Klein GQDs, bound-state GQDs and edge-terminated GQDs according to their different confinement strength. Based on the realization of quasi-bound states in Klein GQDs, external perpendicular magnetic field is utilized as a manipulation approach to trigger and control the novel properties by tuning Berry phase and electron–electron (e–e) interaction. The tip-induced edge-free GQDs can serve as an intuitive mean to explore the broken symmetry states at nanoscale and single-electron accuracy, which are expected to be used in studying physical properties of different two-dimensional materials. Moreover, high-spin magnetic ground states are successfully introduced in edge-terminated GQDs by designing and synthesizing triangulene zigzag nanographenes.     

Controlling the Goos-H?nchen Shift via Incoherent Pumping Field and Electron Tunneling in the Triple Coupled InGaAs/GaAs Quantum Dots

We study the controlHng of the Goos-H?nchen(GH) shifts in reflected and transmitted light beams in the triple coupled InGaAs/GaAs quantum dot(QD) nanostructures with electron tunneling and incoherent pumping Geld.It is shown that the lateral shift can become either large negative or large positive,which can be controlled by the electron tunneling and the rate of incoherent pump Geld in different incident angles.It is also demonstrated that the properties of the GH shifts are strongly dependent on the probe absorption beam of the intracavity medium due to the switching from superluminal Hght propagation to subluminal behavior or vice versa.Our suggested system can be considered as a new theoretical method for developing a new nano-optoelectronic sensor.     

The interaction of ions and easily ionized species with oxide surfaces as studied by tunneling spectroscopy

The thin film properties of alumina and magnesia, the substrates most frequently used in tunneling spectroscopy, are reviewed. The composite barriers, Al₂O₃/SiHO_x and Al_xSO_y are also discussed. Tunneling studies provide information about absorbate-surface interactions; weak counterion adsorption effects through very strong direct chemical modifications can be studied. Tunneling studies of anionic transition metal complexes adsorbed on alumina provide significantly more information about the nature of the surface species than corresponding IR studies. The adsorption of TCNE on alumina from solution is used to exemplify how tunneling spectroscopy may be integrated into a manifold of conventional techniques (IR, ESR, and elution studies in this case).     

Molecule-Dependent Quantum Yield in Photon Emission Scanning Tunneling Microscopy of Mixed Amphiphile Monolayers on Au(111)

Contrast in photon yield between phase-separated domains of reduced and photo-oxidized alkanethiols comprising a mixed molecular monolayer on Au(111) was observed using an ultrahigh vacuum scanning tunneling microscope. The photon quantum yield of the two surface species was measured to be independent of tunneling gap width, within experimental uncertainty, suggesting a novel photon emission mechanism. The spatial resolution of the photon maps was 20 nm. These results indicate that photons emitted by a scanning tunneling microscope reveal chemical information with nanometer spatial resolution on molecularly heterogeneous surfaces.     

半导体三量子点中的多透明窗口及弱光孤子的调控

考虑半导体量子点间隧穿耦合效应,研究非对称半导体三量子点分子中的弱探测光的传播特性。线性情况下,由于点间隧穿耦合和外部控制光的协同调控,探测光的吸收特性将出现共振吸收、隧穿诱导透明单窗口、隧穿诱导透明双窗口及隧穿诱导透明三窗口的转变。此外,从反常色散到正常色散的开关效应可通过改变隧穿强度及光学控制场强度来实现。对于非线性情况,发现孤子的振幅随着点间隧穿耦合系数增大呈先增大再减小随即再次增大并减小的波动变化趋势且出现最大振幅及其对应的点间隧穿耦合强度随着外部控制光场的增大而减小。此外,发现孤子的群速度随着耦合强度的增加呈逐渐减小的趋势。     

Surface dynamics studied by time-dependent tunneling current

Scanning tunneling microscopy (STM) is not only an excellent tool for the study of static geometric structures and electronic structures of surfaces due to its high spatial and energy resolution, but also a powerful tool for the study of surface dynamic behaviors, including surface diffusion, molecular rotation, and surface chemical reactions. Because of the limitation of the scanning speed, the video-STM technique cannot study the fast dynamic processes. Alternatively, a time-dependent tunneling current, I-t curve, method is employed in the research of fast dynamic processes. Usually, this method can detect about 1000 times faster dynamic processes than the traditional video-STM method. When placing the STM tip over a certain interesting position on the sample surface, the changing of tunneling current induced by the surface dynamic phenomena can be recorded as a function of time. In this article, we review the applications of the time-dependent tunneling current method to the studies of surface dynamic phenomena in recent years, especially on surface diffusion, molecular rotation, molecular switching, and chemical reaction.     

能量与空间分辨的单分子显微术--透过碳笼"看"富勒烯包合物内的金属原子

文章作者用探测扫描隧道电流微分谱的方法，对金属富勒烯包合物分子进行了研究，得到了Dy@C82同分异构体Ⅰ的金属-碳笼杂化态在实空间的能量分布图，通过将实验与理论模拟的结果进行比较，推断出Dy原子在碳笼中的位置以及金属富勒烯包合物分子在衬底表面的吸附取向．这项技术为单分子纳米器件的表征和诊断提供了新的途径．     

Controlling the charge state of a single redox molecular switch

Scanning tunneling microscopy and dynamic force microscopy in the noncontact mode are used in combination to investigate the reversible switching between two stable states of a copper complex adsorbed on a NaCl bilayer grown on Cu(111). The molecular conformation in these two states is deduced from scanning tunneling microscopy imaging, while their charge is characterized by the direct measurement of the tip-molecule electrostatic force. These measurements demonstrate that the molecular bistability is achieved through a charge-induced rearrangement of the coordination sphere of the metal complex, qualifying this system as a new electromechanical single-molecular switch.     

Coupled quantum dots: manifestation of an artificial molecule

Transport spectroscopy reveals the microscopic features of few-electron quantum dots which justify the nameartificial atoms. New physics evolve when two quantum dots are coupled by a tunneling barrier. We study, both theoretically and experimentally, the tunneling spectroscopy on a double quantum dot. A detailed lineshape analysis of the conductance resonances proves that off-resonant coherent interdot tunneling governs transport through this system, while tunneling into the double quantum dot occurs resonantly. This coherent interdot tunneling witnesses the evolution of a delocalized electronic state which can be compared to a valence electron of thisartificial molecule.