Nanosphere lithography (NSL) is an inexpensive, simple to implement, inherently parallel, high throughput, materials general nanofabrication technique capable of producing an unexpectedly large variety of nanoparticle structures and well-ordered 2D nanoparticle arrays. This article describes our recent efforts to broaden the scope of NSL to include strategies for the fabrication of several new nanoparticle structural motifs and their characterization by atomic force microscopy. NSL has also been demonstrated to be well-suited to the synthesis of size-tunable noble metal nanoparticles in the 20−1000 nm range. This characteristic of NSL has been especially valuable for investigating the fascinating richness of behavior manifested in size-dependent nanoparticle optics. The use of localized surface plasmon resonance (LSPR) spectroscopy to probe the size-tunable optical properties of Ag nanoparticles and their sensitivity to the local, external dielectric environment (viz., the nanoenvironment) is discussed in...

Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics

Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces.

Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.

https://science.sciencemag.org/content/sci/248/4954/454.full.pdf

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

Nanosphere lithography (NSL) is an inexpensive, inherently parallel, high-throughput, and materials-general nanofabrication technique capable of producing well-ordered 2D periodic particle arrays of nanoparticles. This paper focuses on the synthesis of size-tunable silver nanoparticle arrays by nanosphere lithography and their structural characterization by atomic force microscopy (AFM). The in-plane diameter, a, of Ag nanoparticles was tuned from 21 to 126 nm by systematic variation of the nanosphere diameter, D. Similarly, the out-of-plane height, b, was tuned from 4 to 47 nm by varying the mass thickness, dm, of the Ag overlayer. Experimental measurements of a, b, and interparticle spacing dip of many individual nanoparticles as a function of D and dm were carried out using AFM. These studies show (i) b = dm, (ii) dip accurately corresponds to predictions based on the nanosphere mask geometry, (iii) a, after correction for AFM tip convolution, is governed only by the mask geometry and the standard devi...

Nanosphere Lithography: Size-Tunable Silver Nanoparticle and Surface Cluster Arrays

The chemical modification of hydrogen‐passivated n‐Si (111) surfaces by a scanning tunneling microscope (STM) operating in air is reported. The modified surface regions have been characterized by STM spectroscopy, scanning electron microscopy (SEM), time‐of‐flight secondary‐ion mass spectrometry (TOF SIMS), and chemical etch/Nomarski microscopy. Comparison of STM images with SEM, TOF SIMS, and optical information indicates that the STM contrast mechanism of these features arises entirely from electronic structure effects rather than from topographical differences between the modified and unmodified substrate. No surface modification was observed in a nitrogen ambient. Direct writing of features with 100 nm resolution was demonstrated. The permanence of these features was verified by SEM imaging after three months storage in air. The results suggest that field‐enhanced oxidation/diffusion occurs at the tip‐substrate interface in the presence of oxygen.

Modification of hydrogen-passivated silicon by a scanning tunneling microscope operating in air

We consider the influence of tip-induced band bending on the apparent barrier height deduced from scanning tunneling microscopy (STM) experiments at unpinned semiconductor surfaces. Any voltage applied to a probe tip appears partly in the vacuum gap as an electric field at the semiconductor surface and partly in the semiconductor interior as band bending. The fraction appearing in each region is a function of gap spacing so that modulation of the tip-sample separation inevitably modulates the induced surface potential in the semiconductor. At finite temperature, the height and shape of this barrier determine the probability that an electron will reach the semiconductor surface where it can subsequently tunnel through the vacuum gap. Since the surface potential decreases with increasing tip-sample separation, STM measurements of the tunneling barrier at unpinned semiconductor surfaces will yield unusually low values. Detailed numerical calculations of the effect for passivated n-type Si(111) show it to be of observable magnitude. This mechanism may be distinguished from other recently proposed barrier-lowering mechanisms in that it is doping dependent, potentially long range, and possesses a unique voltage signature.

/pdf/band-bending-and-the-apparent-barrier-height-in-scanning-3bmymxgg26.pdf

Band bending and the apparent barrier height in scanning tunneling microscopy

We have been successful in obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current–voltage (I–V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. Our observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.

/pdf/imaging-graphite-in-air-by-scanning-tunneling-microscopy-4vl1istwkj.pdf

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

Molybdenum disulfide, a layered semiconductor, is an interesting material to study with the tunneling microscope because two structurally and electronically different atomic species may be probed at its surface. We report on a vacuum scanning tunneling microscopy study of 2H-MoS2. Atomic resolution topographs and current images show the symmetry of the surface unit cell and clearly reveal two distinct atomic sites in agreement with the well-known x-ray crystal structure.

/pdf/tunneling-microscopy-of-2h-mos2-a-compound-semiconductor-1oxaw2r3w0.pdf

Tunneling microscopy of 2H-MoS2: A compound semiconductor surface.

Scanning tunneling microscopy (STM) has been enormously successful in solving several important problems in the geometric and electronic structure of homogeneous metallic and semiconducting surfaces. A central question which remains to be answered with respect to the study of compound surf~ces, however, is tpe extent to which the chemical identity of constituent at~~s may be established. Recently, progress in this area was made by Feenstra et al. who succeeded in selectively imaging either Ga or As atoms on the GaAs ( 110) surface. 2 So far this is the only case where such selectivity has been achieved. In an effort to add to our understanding of compound surface imaging we have undertaken a vacuum STM study of 2H-MoS2, a material which has two structurally and electronically different atomic species at its surface. Because it is semiconducting, one expects the surface electronic wave functions of molybdenum disulfide to be preferentially localized over specific atoms. We find that it is possible to distinguish two distinct atomic sites at this surface by tunneling microscopy, both in the conventional constantcurrent variable-height mode (without obtaining anomalously large corrugation amplitudes) as well as in the variable-current constant-height mode. An example is presented in Fig. 1 and a more detailed account of our work is given elsewhere. 3

Summary Abstract: Scanning tunneling microscopy investigation of 2H‐MoS2: A layered semiconducting transition‐metal dichalcogenide

The surface of silver containing salt of bis(ethylenedithio)tetrathiafulvalene (BEDT‐TTF) was studied with the computer‐controlled scanning tunneling microscope developed in our laboratory. The crystal surface of the charge‐transfer complex is well ordered and a regular array of corrugations is clearly visible. The prominent feature of the experimental scanning tunneling microscopy images is in agreement with the bulk crystal structure obtained by x‐ray diffraction method.

/pdf/scanning-tunneling-microscopy-of-silver-containing-salt-of-3mcimpn7u6.pdf

J. D. Baldeschwieler

Papers

Band bending and the apparent barrier height in scanning tunneling microscopy

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

Tunneling microscopy of 2H-MoS2: A compound semiconductor surface.

Summary Abstract: Scanning tunneling microscopy investigation of 2H‐MoS2: A layered semiconducting transition‐metal dichalcogenide

Scanning tunneling microscopy of silver containing salt of bis(ethylenedithio)tetrathiafulvalene