One-dimensional structural and electronic properties of magnetite Fe3O4(1 1 0)

We study the structure and the electronic properties of the (1 1 0) surfaces of magnetite Fe₃O₄ thin films by scanning tunneling microscopy (STM) and spectroscopy (STS). The STM images show a surface reconstruction consisting of ridges along the direction. Based on atomically resolved STM images we present a model for the observed ridge reconstruction of the surface, in agreement with a bulk-truncated layer containing both octahedral and tetrahedral iron ions. The metallic and semiconductor-like shapes of the measured current-voltage (I-V) curves indicate a non-uniform segregation of magnesium through the film. The weak contrast between the tops and valleys of ridges measured in the STS current maps is attributed to tetrahedral and octahedral coordination at the tops and the valleys, respectively. This attribution is in agreement with the proposed structure model. We observe a contrast enhancement at a tip change accompanied by a corrugation enhancement. This tip change is induced by picking up material from the sample, resulting in a magnetic tip. Thus, the contrast enhancement is attributed to detection of spin polarized current.     

Molecular resolution of Langmuir-Blodgett monolayers on tungsten diselenide by scanning tunneling microscopy

Langmuir-Blodgett (LB) monolayers of 22-tricosenoic acid deposited on layered tungsten diselenide (WSe₂) have been investigated by Scanning Tunneling Microscopy (STM) in air. Reproducible molecular scale imaging of extended areas (16×16 nm) of the films was possible over more than two months after preparation. This indicates a high stability of the films on this type of polar substrates. The two-dimensional lattice of the organic film is incommensurate to the lattice of the WSe₂ substrates. The STM experiments also indicate that the tip partially penetrates the organic film without destroying the film structure.     

Metal enhanced photoluminescence of near-infrared CdTexSe1−x quantum dots

High-quality alloyed near-infrared CdTe_xSe_1?x quantum dots were synthesized by a modified organometallic method. The emission wavelength of the alloyed quantum dots were turned from visible to near-infrared range by changing the composition of the precursor. The photoluminescence intensity of the alloyed quantum dots was further enhanced by coupling through localized surface plasmons from Au nanoparticles. The alloyed CdTeSe quantum dots coupled with Au nanoparticles exhibited a 4 times photoluminescence enhancement than that of bare CdTeSe quantum dots by turning the localized surface plasmons resonant absorption of Au nanoparticles consistent to the excitation wavelength. This method will be beneficial for the potential applications in the biological imaging and detection.     

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.     

Tunneling experiments involving magnetic tip and magnetic sample

Preliminary STM experiments have been performed on a Fe polycrystal using a CoCr magnetic tip. With the usualW-tip, the Fe topography reveals a typical polycrystalline corrugation. The CoCr tip was obtained by cleaving a Si wafer covered with a CoCr film. Its operation was verified to be normal on a Cu single-crystal surface. The combination of Fe sample and CoCr tip leads to a strongly enhanced corrugation as compared to the normal topography. A preliminary explanation, in terms of force enhancement, is based on the interaction of the tip magnetic moment with the field gradient near the sample surface.     

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO₂(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO_2−x towards Ti₂O₃.     

Photon mapping of MgO thin films with an STM

The light emission from an STM junction consisting of an MgO thin film on Mo(0 0 1) and an Au tip is analyzed with respect to its spatial distribution for various excitation conditions. The spectral characteristic of the light is compatible with an emission mechanism mediated by tip-induced plasmons that are excited by inelastic electron tunneling involving field-emission resonances in the tip-sample gap. The dependence of field-emission resonances on the MgO work function allows the controlled stimulation of differently thick oxide islands in the photon maps by changing the sample bias.     

The structure of ordered Au films on TiOx

The growth of Au on an ultra-thin, ordered Mo(1 1 2)-(8 × 2)-TiO_x, was investigated using scanning tunneling microscopy (STM), low energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). Wetting of the TiO_x surface by Au was observed with STM and LEISS, and the ordering of the Au films was atomically resolved with STM. TPD showed that Au binds more strongly to the reduced TiO_x film than to bulk TiO₂, but more weakly than to the Mo substrate. The Au-TiO_x binding energy is greater than Au-Au in bulk Au. The oxidation state of Ti in the TiO_x film was deduced by XPS and from the Ti-O phonon shifts relative to bulk TiO₂. The TiO_x/Mo(1 1 2) film structure and those for the (1 × 1)- and (1 × 3)-Au/TiO_x/Mo(1 1 2) surfaces are discussed.     

STM-induced photon emission spectroscopy of granular gold surfaces in air

Light emission has been detected under ambient conditions in the tip–sample region of a scanning tunneling microscope (STM) consisting of an etched gold tip and a granular gold film. The photon yield as a function of surface geometry (photon mapping) has been studied. By means of STM, it was possible to measure photon emission spectra locally. We have studied the effect of grain size and applied bias voltage on the spectrum. We found that the peak position in the photon emission spectrum shifts to a shorter wavelength when increasing the bias voltage and shifts to a longer wavelength when tunneling to larger grains. These effects can be understood in a simple model which considers tunneling electrons exciting localized surface plasmons which decay by emitting photons.     

纳腔等离激元对分子上和分子旁边的隧穿电子诱导发光的作用研究

本文使用含时量子主方程,从理论上计算了当分子或者等离激元分别被激发的情形下等离激元-分子耦合体系发光特性的时间和光谱演化,并在此基础上讨论了纳腔等离激元在扫描隧道显微镜（STM）诱导发光中发挥的不同作用.当STM针尖在分子上方,隧穿电子可以直接激发分子时,纳腔等离激元的主要作用是通过提高分子的辐射速率来增强其发光,此时耦合体系表现出具有分子特征的尖锐发光峰.另一方面,当STM针尖非常靠近分子边缘但没有载流子注入直接激发分子时,等离激元-分子之间的相干耦合会在这两个量子客体之间产生相消干涉,导致在分子激子能量附近出现法诺共振,使我们观察到具有法诺凹谷特征的等离激元发光光谱.     

Bias dependence of apparent layer thickness and Moiré pattern on NaCl/Cu(001)

Bias-dependent features of the insulating NaCl layer grown on Cu(001) have been investigated by scanning tunneling microscopy/spectroscopy (STM/STS). The apparent layer thickness of the NaCl film is variable at bias voltages ranging from 2.8 to 3.2 V as well as from 4.0 to 5.0 V, and the Moiré pattern induced by NaCl–Cu lattice mismatch also shows bias dependence. The z–V (dz/dV–V) curves and dI/dV mapping measurements reveal that the resonant tunneling between the image potential states (IPSs) on Cu(001) and the Fermi level of the STM tip leads to drastic variations of these features.     

Experimental study of the enhanced Raman scattering in the Sarid-type 4-layered attenuated total reflection configurations

We experimentally explored the enhanced Raman scattering in pyridine (C₅H₅N) due to the excitation of long range surface plasmons (LRSP's) guided by a symmetrically bounded silver slab in the Sarid-type ATR (attenuated total reflection) configuration. We have observed that, with decreasing the metallic thickness, the absolute enhancement of the Raman cross section increases while the width of the polar angular distribution of the signal decreases. This observed dependence of the enhanced Raman intensity on the thickness of the thin Ag slab and the collecting solid angle is well explained by assuming that the Raman scattering is mediated by LRSP's. The observed maximum enhancement factor is 4 × 10⁶ for the 140 Å silver slab which agrees well with the theoretically predicted value of 6 × 10⁶.     

Manipulation of organic “needles” using an STM operated under SEM control

Nano-sized P4P (para-quaterphenylene) organic crystallites grown on a gold substrate were removed and redeposited using the tip of an STM. The controlled lift-off of the needle-shaped crystallites was imaged by a simultaneously operated SEM. The forces exerted by the metal STM tip on the nano-crystal during this nano-manipulation process could be determined by monitoring the deflection of the STM tip. After the lift-off process the former contact area was imaged using the STM. These STM micrographs clearly demonstrate that the region between the needles is covered by a P4P layer with a thickness of at least 3 nm.     

外电场下STM钨针尖电子结构的理论研究

采用离散变分局域密度泛函方法，研究了外电场对钨(111)面针尖电子结构的影响.详细计算和分析了在不同偏压和距离条件下，钨针尖的隧道激活轨道和电荷分布.研究结果表明：隧道激活轨道中针尖原子的成分对外偏压的极性、大小以及针尖与样品之间的距离都较敏感.与过去理论计算结果不同，钨针尖原子的5d_z²轨道对隧道激活轨道有一定贡献，但并不是最主要的.在加正偏压时对隧道激活轨道贡献最大的为5d_xz和5d_yz轨道，而在加负偏压时则为包括 关键词：     

Scanning tunnelling microscopy of charge-density waves in transition metal chalcogenides

We have used scanning tunnelling microscopes (STMs) operating at liquid helium and liquid nitrogen temperatures to image the charge-density waves (CDWs) in transition metal chalcogenides. The layer structure dichalcogenides TaSe₂, TaS₂, NbSe₂, VSe₂, TiSe₂ and TiS₂ have been studied including representative polytype phases such as 1T, 2H and 4Hb. Experimental results are presented for the complete range of CDW amplitudes and structures observed in these materials. In most cases both the CDW and the surface atomic structure have been simultaneously imaged. Results on the trichalcogenide NbSe₃ are also included.

The formation of the CDW along with the associated periodic lattice distortion gaps the Fermi surface (FS) and modifies the local density-of-states (LDOS) detected by the tunnelling process. The tunnelling microscopes have been operated mostly in the constant current mode which maps the LDOS at the position of the tunnelling tip. The relative amplitudes and profiles of the CDW superlattice and the atomic lattice have been measured and confirm on an atomic scale the CDW structures predicted by X-ray, electron and neutron diffraction. The absolute STM deflections are larger than expected for the CDW induced modifications of the LDOS above the surface and possible enhancement mechanisms are reviewed.

In the 2H trigonal prismatic coordination phases the CDWs involve a relatively small charge transfer and the atomic structure dominates the STM images. In the 1T octahedral coordination phases the charge transfer is large and the CDW structure dominates the STM image with an anomalously large enhancement of the STM profile. Systematic comparison of the STM profiles with band structure and FS information is included.

In the case of the 4Hb mixed coordination phases at the lowest temperatures two nearly independent CDWs form in alternate sandwiches. STM studies on 4Hb crystals with both octahedral and trigonal prismatic surface sandwiches have been carried out. The STM scans detect the relative strengths of the two CDWs as well as the interactions between the two types of CDW structure.

The STM scans are also able to detect defects and domain structure in the CDW image. Several examples will be given demonstrating the potential of the STM to detect these local variations in LDOS on an atomic scale. In contrast to the layer structure crystals the linear chain compound NbSe₃ shows a complex surface atomic structure as well as the formation of two CDWs. The surface atomic structure is resolved in the STM scans and profiles have detected the presence of the CDW modulation at 77K and 4.2K. These results demonstrate the feasibility of detecting CDW structure in the presence of complex atomic structure and using materials where dynamical CDW effects can also be studied by STM.

The range of STM results presented here show that the STM scans are extremely sensitive to the detail of the CDW structure and its effect on the LDOS. Although much of this structure has been deduced from diffraction studies, the ability to examine the CDW structure on an atomic scale with the STM is new. The sensitivity of the STM method suggests potential applications to a wide range of electronic structures in materials.     

Adiabatic nano-focusing of plasmons by sharp metallic wedges

In this paper, we demonstrate the possibility of efficient adiabatic nano-focusing of plasmons by a sharp triangular metal wedge. The geometrical optics approach and the approximation of continuous electrodynamics are used for the analysis. In particular, it is demonstrated that both the phase and group velocities of an incident anti-symmetric (with respect to the magnetic field) plasmon tend to zero at the tip of the wedge, and the plasmon adiabatically slows down, eventually dissipating in the metal. Typically, the amplitude of the plasmon significantly increases near the wedge tip, but this increase is finite even in the absence of dissipation in the metal. The dependence of the local field enhancement near the tip on structural parameters, dissipation in the metal, angle of incidence, etc., is analyzed in this paper. It is also shown that an anti-symmetric film plasmon can effectively be guided by a triangular metal wedge, forming a wedge plasmon mode that is localized near the tip of the wedge and propagates along this tip. A new existence condition for these localized wedge plasmons is derived and discussed. PACS 78.67.-n; 68.37.Uv; 73.20.Mf     

Atomic resolution in dynamic force microscopy across steps on Si(1 1 1)7×7

In this note we report the first observation of salient features of the Si(1 1 1)7×7 reconstructed surface across monatomic steps by dynamic atomic force microscopy (AFM) in ultrahigh vacuum (UHV). Simultaneous measurements of the resonance frequency shift Δf of the Si-cantilever and of the mean tunneling current ī _i from the cleaned Si tip indicate a restricted range for stable imaging with true atomic resolution. The corresponding characteristics vs. distance reveal why feedback control via Δf is problematic, whereas it is as successful as in conventional STM via ī _i .     

A large-area photolithography technique based on surface plasmons leakage modes

An atomic force microscope (AFM) assisted surface plasmons leakage radiation photolithography technique has been numerically demonstrated by using two-dimensional finite-difference time-domain (2D-FDTD) method. With the aid of a metallic AFM tip, particular characteristic of the Kretstchmann configuration to excite surface plasmons (SPs) is utilized to achieve large-area patterns with high spatial resolution and contrast, the photoresist could be exposed with low power laser due to the remarkable local field enhancement at the metal/dielectric interface and the resonant localized SPs modes near the tip. Good tolerance on the film thickness and incident angle has been obtained, which provides a good practicability for experiments. This photolithography technique proposed here can realize large-area, high-resolution, high-contrast, nondestructive, arbitrary-structure fabrication of nanoscale devices.