Introduction to Scanning Tunneling Microscopy
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Volume 16 , Issue 2 March—April Pages Related Information. Close Figure Viewer. Browse All Figures Return to Figure. Previous Figure Next Figure. Email or Customer ID. Forgot password? Old Password. New Password. The area is approximately 10X10 nm2.
Tip bias was 1 nA at 0. This implies an imaging mechanism whereby the tunneling sites inherent in the substrate are enhanced by the presence of the intermediate organic layer. See McGonigal, Bernhardt, and Thomson for details. Original image courtesy of D. Atomic force microscopy AFM enables the direct investigation of electrically insulating surfaces.
These two images of typical insulators, rock salt and mica, were obtained with an AFM using the opticalbeam-deflection method, operating in the short-range repulsive-force regime. Atomic structures are clearly resolved. See Meyer and Amer for details. Original images courtesy of N. Overshadowed by the more interesting images of Si lll -7X7 obtained in Fall , the gold image was not submitted for publication until April see Binnig and Rohrer, See, for example, Berndt et al. Today, atomic resolution on rigid surfaces has become a "must" in STM experiments Rohrer, Atomic resolution on Au l 11 was first observed by Chiang et al.
Using a tip treatment procedure as described by Wintterlin et al. Often, the corrugation is reversed, that is, the atomic sites appear as depressions instead of protrusions. In the lower image, the atomic arrangement near a step is resolved. See Barth et al. On a large scale, three equivalent orientations for this reconstruction coexist on the surface. Furthermore, on an intermediate scale, a herring-bone pattern is formed. Original image be courtesy of J.
Introduction to Scanning Tunneling Microscopy
Nucleation and growth of Ni and Fe on Au lll. Each dot is a monolayer-high island containing atoms, b Close-up STM image of the Fe islands. The Fe islands decorate domain boundaries on elbows of the herring-bone pattern to form a regular pattern. See Chambliss et al. Original images courtesy of D. Tip induced modification of the herringbone structure on Au lll. The herringbone structure is a result of a subtle balance of energy and stress on the Au lll surface which involves a large number of atoms.
Using a voltage pulse to create a hole on the Au l 11 surface, the herringbone pattern deforms accordingly.
C. Julian Chen. The scanning tunneling microscope (STM) and the atomic force microscope (AFM), both capable of visualizing and manipulating individual atoms, are the cornerstones of nanoscience and nanotechnology today. Keywords: nanoscience, nanotechnology, scanning tunneling. Introduction to Scanning. Tunneling Microscopy. Second Edition. C. Julian Chen. Department of Applied Physics and Applied Mathematics. Columbia University.
The evolution of the pattern is observed by STM in real time, a Before making a hole, b After the hole was formed, c About 3 minutes after the hole was formed, d After about 9 minutes, e After about 15 minutes, f After 50 minutes. See Hasegawa and Avouris for details.
Original images courtesy of H. Hasegawa and Ph. By applying a voltage pulse between a gold tip and a gold surface, a mound of A in diameter and A in height is formed. The location of the mound can be precisely controlled. By programming the positions of the mounds, a gold map is constructed. The diameter of the map is about 1 xm, giving the map a scale of about 10 trillion to 1. For the deposition process, see Mamin, Guenter, and Rugar for details.
Original image courtesy of H. Figure 1.
Scanning tunneling microscope (STM)
A probe tip, usually made of W or Pt-Ir alloy, is attached to a piezodrive, which consists of three mutually perpendicular piezoelectric transducers: x piezo, y piezo, and z piezo. Upon applying a voltage, a piezoelectric transducer expands or contracts. By applying a sawtooth voltage on the x piezo and a voltage ramp on the y piezo, the tip scans on the xy plane. Using the coarse positioner and the z piezo, the tip and the sample are brought to within a few angstroms of each other.
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The electron wavefunctions in the tip overlap electron wavefunctions in the sample surface. A bias voltage, applied between the tip and the sample, causes an electrical current to flow. Such a current is a quantum-mechanical phenomenon, tunneling, which is explained briefly in Section 1. Schematic diagram of the scanning tunneling microscope. The difference is then amplified again to drive the z piezo.
The phase of the amplifiers is chosen to provide negative feedback: If the tunneling current is larger than the reference value, then the voltage applied to the z piezo tends to withdraw the tip from the sample surface, and vice versa. Therefore, an equilibrium z position is established through the feedback loop. As the tip scans over the xy plane, a two-dimensional array of equilibrium z positions, representing a contour plot of the equal tunneling-current surface, is obtained and stored.
The contour plot is displayed on a computer screen, either as a line-scan image or as a gray-scale image Fig. The line-scan image is a sequence of curves, each of which represents a contour along the x direction with constant v. The curves with different ys are displaced vertically. The gray-scale image is similar to a black-and-white television picture. The bright spots represent high z values protrusions , and the dark spots represent low z values depressions. To achieve atomic resolution, vibration isolation is essential.
There are two ways to achieve a suitable solution. The first is to make the STM unit as rigid as possible. The second is to reduce the transmission of environmental vibration to the STM unit. A commonly used vibration isolation system consists of a set of suspension springs and a damping mechanism. The STM experiments can be performed in a variety of ambiences: in air, in inert gas, in ultrahigh vacuum, or in liquids, including insulating and cryogenic liquids, and even electrolytes.
The operating temperature ranges from absolute zero Line-scan image and gray-scale image. The two images represent the same set of data collected by STM on a S i l l l - 7 X 7 surface with steps. Dimensions: X A2. Reproduced from Becker et al. In classical mechanics, an electron with energy E moving in a potential U z is described by where m is the electron mass, 9.
On the other hand, the electron cannot penetrate into any region with E Consider the case of a piecewise-constant potential, as shown in Fig. The difference between classical theory and quantum theory.
In quantum mechanics, an electron has a nonzero probability of tunneling through a potential barrier. After Van Vleck; see Walmsley, In the classically forbidden region, Eq. Starting from this elementary model, with a little more effort, we can explain some basic features of metal-vacuum-metal tunneling, as shown in Fig.
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In general, the work function depends not only on the material, but also on the crystallographic orientation of the surface see Section 4. The work functions for alkali metals are substantially lower, typically eV. Neglecting the thermal excitation, the Fermi level is the upper limit of the occupied states in a metal.
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To simplify discussion, we assume that the work functions of the tip and the sample are equal. The electron in the sample can tunnel into the tip and vice visa. However, without a bias voltage, there is no net tunneling current. Table 1. A one-dimensional nietal-vacuum-metal tunneling junction. The sample, left, and the tip, right, are modeled as semi-infinite pieces of free-electron metal. By applying a bias voltage V, a net tunneling current occurs. We assume that the bias is much smaller than the value of the work function, that is, eV.
Using eV as the unit of the work function, and A"1 as the unit of the decay constant, the numerical value of Eq. During a scan, the condition of the tip usually does not vary. The electrons coming to the tip surface, z — W, have a constant velocity to flow into the tip. This number depends on the local nature of the sample surface. For metals, it is finite. For semiconductors and insulators, the number is very small or zero. For semimetals, it is in between. By including all the sample states in the energy interval eV, the tunneling current is If V is small enough that the density of electronic states does not vary significantly within it, the sum in Eq.
The LDOS is the number of electrons per unit volume per unit energy, at a given point in space and at a given energy. It has a nice feature as follows.