Designing an Air Scanning Tunneling Microscope to Study Two-Dimensional Materials

1. Introduction

The scanning tunneling microscope (STM) has developed into an imaging method with diverse possibilities for real-space imaging on a scale that extends to atomic dimensions. The interaction that is monitored in STM is the tunneling current between a metallic tip and a conducting substrate, which are in very close proximity but not actually in physical contact. It is quantum mechanical tunneling that permits the electrons to tunnel through the potential barrier, which they could not surmount according to the classical laws of physics. In this model, the probability of tunneling is exponentially dependent upon the distance of separation between the tip and surface: the tunneling current is therefore a very sensitive probe of this separation. Another great advantage of a STM is that it may allow the characterization and identification of individual atoms and may lead to the manipulation of materials at the atomic scale. One of the principal reasons for building this air STM is to be able to study two-dimensional materials such as graphene and T1S2. Due to the electronic properties of these materials it has been of great interest to explore their use in electronic device applications such as FETs.

Because of the reactivity that many surfaces, such as Si have in ambient conditions, the use of an ultra-high vacuum (UHV) system with pressures <10^-9torr is necessary to study those surfaces with a STM. However, since only materials that are unreactive in ambient conditions will be studied, it was sufficient to design and assemble an air STM in order to characterize materials such us graphene, which do not have to be constrained to an UHV environment. One of the principal advantages of using an air STM rather than an UHV-STM is the significant time reduction in transferring and preparing a sample for imaging. While it can take up to two or three days to prepare a sample for imaging in UHV, a sample can be prepared for imaging in ambient conditions in just a few minutes. Also, an air STM is relatively cheap and involves a simple design, which ameliorates maintenance procedures.

Due to the proximity between the tip and the sample (~ 5A), main considerations in the design for this air STM were the vibration and noise isolation of the entire system, the coarse and fine approach of the tip probe to the sample, and the electrical wiring connections. Electronic equipment from RHK Technology was used to control the STM. A detailed view of the design-assembly will be explained in the next section of this article.

2. Experimental Design

Coarse and Fine Approach Design

An air STM with an axially symmetric structure was designed and built in our laboratory. The use of a commercially available micromanipulator (from Vacuum Generators Limited, Hastings, England) was used to perform the vertical coarse approach of the STM head, as well as the horizontal coarse translation along the x and у axes (see Fig. 1). After the coarse approach of the STM head, the head becomes decoupled from the head holder (see Fig. 2). This helps to avoid vibrational noise transmission to the STM tip. In order to perform the fine adjustment of the tip position, a set of three ceramic piezoelectric tubes (Staveley Sensors, Inc., EBL#2 with Ni electrodes) were used to displace the decoupled STM tip, via a ramp mechanism at an approximate rate of 800nm/s to 3 µm/s, see Figs. 2(a) and (b). The dimensions of the piezoelectric tubes are as follows: 0.125" outer diameter x 0.5" length x 0.020" wall thickness. Appropriate voltages applied to the electrodes cause expansion or contraction of the tube (z motion) or bending in two orthogonal directions (A and В motion). This phenomenon is known as the reverse piezoelectric effect. The outside of the piezo tube is separated into five quadrants. The first one attaches to the nickel-plated aluminum body, which is connected to facilitate the grounding connection for all the piezo tube-legs. The other quadrants are used to move the piezo tube-legs, providing either rotation down or displacement across the ramp.

Figure 1. Schematic of electrical connections at the STM head, (bottom view) showing piezo quadrants. L labels the piezo legs and T corresponds to the tip probe. X and Y correspond to connections controlling tip displacement in the plane of the sample surface. A and В correspond to connections controlling rotation of the STM head up and down the ramp.

In order to ensure the samples are properly elevated within the range of tip approach, which is dictated by the ramp for the piezoelectric legs, a set of Ni foils (~ 1 mm each) were used because it is conducting and an available material in our lab. The principal reason for using these foils is because they are sufficiently thin to allow fairly precise control of the sample's height by stacking them together. Each of the Ni foils was attached to one another using graphite tape to avoid displacement between foils. To fix the sample's position, a set of clamps was used, which also promoted better electrical connection between the sample and the ramp's base (see Fig. 3).

Figure 2. Photograph of the air STM. A) Complete view of the STM-frame and micromanipulator. The total height is 26 in. b) Closer view of the STM piezo-ramp assembly.

Figure 3. Top-view photograph of the assembly including the ramp, sample, and clamps. The sample shown is HOPG.

Tip Probe

A stable, conducting material that can be formed into a sharp point (ideally 1 atom) is needed for the tip. The most common materials for tip probes are tungsten and Pt/Ir. Commercially available Pt/Ir tips (Nanoprobe™ SPM Tips from Digital Instruments, model PT-ECM) with a total length of 14 mm, were used for this air STM. Pt provides better resistance to oxidation in ambient conditions than tungsten and small amounts of Ir increase the hardness of the tip. The scanning approach mechanism used for this tip was a ceramic piezoelectric material, controlled to have x and у displacements (see Figure 1 and 2b). The tip and the specimen were cleaned only with acetone followed by isopropanol and distilled water. The exposed tip was about 1.5 mm in length. It is ideal to have the exposed tip length as small as possible to avoid vibrational noise.

Electrical Connections and Electronic Design

Electrical connections for the STM head to the piezo tubes and the voltage bias were made with 0.003 in. and 0.010 in. diameter Kapton-coated copper wire, respectively (California Fine Wire, Co.). All wire-wire and wire-pin connections were made using solder and flux (Weller® EC2002M). The electronics used for the computer data input and output was a control module, model V-SCAN 100, and a control panel, model STM 100 from RHK Technology. These electronics are important for surface imaging in one of two ways: 1) in constant height mode, in which the tunneling current is monitored as the tip is scanned parallel to the surface and 2) in constant current mode, in which the tunneling current is maintained constant as the tip is scanned across the surface. In practice, it is most common to image the surface in constant current mode. This is achieved by adjusting the tip's height above the surface so that the tunneling current does not vary with the lateral tip position. For example, in this mode the tip will move slightly upwards as it passes over a surface atom, and conversely, slightly in towards the surface as it passes over a hollow site. This is the major concern of the feedback loop circuit, which is the most critical electronic circuit in the STM design, and is shown in Figure 4.

Figure 4. Feedback loop test diagram, which allows monitoring the stability of the STM-100