Variable-temperature Ultrahigh Vacuum Research Articles

A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope with spiral scan capabilities.

We present the design and development of a variable-temperature high-speed scanning tunneling microscope (STM). The setup consists of a two-chamber ultra-high vacuum system, including a preparation and a main chamber. The preparation chamber is equipped with standard preparation tools for sample cleaning and film growth. The main chamber hosts the STM that is located within a continuous flow cryostat for counter-cooling during high-temperature measurements. The microscope body is compact, rigid, and highly symmetric to ensure vibrational stability and low thermal drift. We designed a hybrid scanner made of two independent tube piezos for slow and fast scanning, respectively. A commercial STM controller is used for slow scanning, while a high-speed Versa Module Eurocard bus system controls fast scanning. Here, we implement non-conventional spiral geometries for high-speed scanning, which consist of smooth sine and cosine signals created by an arbitrary waveform generator. The tip scans in a quasi-constant height mode, where the logarithm of the tunneling current signal can be regarded as roughly proportional to the surface topography. Scan control and data acquisition have been programmed in the experimental physics and industrial control system framework. With the spiral scans, we atomically resolved diffusion processes of oxygen atoms on the Ru(0001) surface and achieved a time resolution of 8.3 ms per frame at different temperatures. Variable-temperature measurements reveal an influence of the temperature on the oxygen diffusion rate.

Intrinsic Nucleation Mechanism and Disorder Effects in Polarization Switching on Ferroelectric Surfaces

The temperature dependence of ferroelectric domain nucleation in epitaxial films of BiFeO3 is studied using variable temperature ultrahigh vacuum piezoresponse force spectroscopy in the 50 to 300 K temperature range. The nucleation bias corresponding to the onset of local ferroelectric switching in the volume of an electrostatic field confined by the metal tip was found to change less than 20% across the entire temperature range. A combination of the analytical and phase-field analysis proves that the weak temperature dependence of nucleation bias is a hallmark of an intrinsic nucleation mechanism with minimal contribution of thermal fluctuations. The effect of disorder on the observed distribution of the nucleation bias between vacuum and ambient environments is compared.

Probing charge transport at the single-molecule level on silicon by using cryogenic ultra-high vacuum scanning tunneling microscopy

A cryogenic variable-temperature ultra-high vacuum scanning tunneling microscope is used for measuring the electrical properties of isolated cyclopentene molecules adsorbed to the degenerately p-type Si(100)-2x1 surface at a temperature of 80 K. Current-voltage curves taken under these conditions show negative differential resistance at positive sample bias, in agreement with previous observations at room temperature. Because of the enhanced stability of the scanning tunneling microscope at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, we show that current-voltage curves on isolated cyclopentene molecules are reproducible and possess negligible hysteresis for a given tip-molecule distance. On the other hand, subsequent measurements with variable tip position show that the negative differential resistance voltage increases with increasing tip-molecule distance. By using a one-dimensional capacitive equivalent circuit and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative estimate for the alignment of the highest occupied molecular orbital of cyclopentene with respect to the Fermi level of the silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the single-molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices.

Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope for single molecule studies on silicon surfaces

The design and performance of a variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) is presented. This STM operates from 8.2 to 300 K in a UHV environment with a base pressure of less than 6×10−11 Torr. Cooling is achieved from 300 to 80 K within 3.5 h and from 80 to 8.2 K within 4.5 h. The base temperature of 8.2 K is maintained at a liquid helium consumption rate of 0.9 l/h. This design allows for direct optical access to the tip-sample interface and direct line-of-sight dosing while the sample is mounted in the STM. The STM tip may be coarse translated laterally in two dimensions through a 6-mm-diam area at all temperatures. With the feedback loop off, the drift in the tip-sample spacing is approximately 0.008 Å/min at 8.2 K. Atomic resolution feedback controlled lithography is performed on hydrogen passivated Si(100) and differential tunneling conductance maps are gathered for isolated cyclopentene molecules on unpassivated Si(100), thus demonstrating that this system is well suited for studying single molecules on silicon surfaces from 8.2 to 300 K.

Cluster critical size effect during growth on a heterogeneous surface

Nucleation and growth on a complex surface containing a regular network of defects is investigated in the case of Co on Au(111). By means of variable temperature ultrahigh vacuum scanning tunneling microscopy it is shown that both monolayer and bilayer Co clusters can be found on the same surface reconstruction, depending on the nature of the particular nucleation site: two types of point dislocations located at the elbows of the herringbone reconstruction. The existence of a critical size for the structural transition from monolayer to the bilayer islands is inferred on the basis of experimental results, quenched molecular dynamics calculations, and Metropolis Monte Carlo simulations. The resulting phase diagram provides a detailed characterization of the different growth modes (monolayer and bilayer islands) as a function of temperature and coverage.

Combined Kerr microscope and magnetic force microscope for variable temperature ultrahigh vacuum investigations

A new system combining a Kerr microscope and a magnetic force microscope for the study of magnetic domains in ultrathin films under ultrahigh vacuum conditions is presented. Due to the overlapping imaging range of both techniques magnetic domains can be investigated over a lateral range from millimeter down to fractions of a micrometer. Experiments can be done at variable temperatures, from 80 K to 600 K. First results are presented showing the same magnetic domain in a three monolayer Fe film on Cu (100) imaged with both techniques.

Submonolayer homoepitaxial growth on Ag(1 1 0)

The submonolayer homoepitaxial growth on Ag(1 1 0) is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The evolution of island density is studied as a function of temperature at given flux (≈0.0025 ML s −1) and coverage (0.16 ML) to evaluate the energy barriers and the binding energies for the in-channel and cross-channel diffusion. The experimental results are compared with kinetic Monte Carlo simulation.

A variable-temperature ultrahigh vacuum scanning tunneling microscope

A variable-temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) was designed and tested. Design details and initial results are presented. The STM is directly attached to the cold face of a continuous flow cryostat which is mounted into a two-chamber UHV system. A significant advantage of this system in comparison to many others is, that samples can be cooled down to base temperature of 6.5 K within very short times of below 2 h. This feature not only increases the potential sample throughput, it also allows to cycle the sample temperature within the regime below 20 K without losing track of given sample locations. The instrument was tested by imaging Au layers on graphite. The vertical stability at low temperature was found to be below 3 pm. Images recorded at 6.5 K show crystalline Au islands and the Au(111)22×√3 reconstruction with atomic resolution. Using a resistive heater, the sample temperature was adjusted between 6.5 and 20 K. After an equilibration time of 15 min, the displacement due to the temperature change remained below 150 nm. Scanning tunneling spectroscopy on Au(111) grains resolves the Au(111) surface state.

Two-Dimensional Phase Diagram of Decanethiol on Au(111)

On the basis of variable-temperature ultrahigh vacuum scanning tunneling microscopy data, we propose a two-dimensional phase diagram of monolayer decanethiol on Au(111). Four triple-point temperatures were determined: T1 at ≈27 °C, T2 at ≈33 °C, T3 at ≈35 °C, and T4 at ≈56 °C. T1 defines the lowest temperature melting point, and T4 defines the temperature above which striped phases are metastable. These data provide a fundamental framework to understand and control mesoscale monolayer structure; moreover, they provide fundamental insight into the two-dimensional phase behavior of molecules with many degrees-of-freedom.

Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope

A cryogenic variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) has been developed. This design utilizes a novel vibration isolation that provides an active thermal link to the cooling source without the standard tradeoff of compromising mechanical isolation. A welded bellows serves as the basis for the vibration isolation system. This bellows houses a heat exchanger which uses helium gas to form a thermal link between the STM and the cryogenic. This STM is fully integrated into a multichamber UHV STM system now in place in our laboratory and atomic resolution operation is demonstrated from 11 to 300 K.

Time evolution of adatom and vacancy clusters on Ag(110)

The time evolution of adatom and vacancy islands on an Ag(110) surface is studied by using a variable-temperature ultrahigh vacuum scanning tunneling microscope. The islands are created by Ar + ion sputtering at low temperatures and at very low fluxes (∼0.004 ML s −1). The area of both kinds of island decreases linearly as a function of time, at least for small islands. The experimental results are compared with kinetic Monte Carlo simulations.

Atomically Resolved Determination of the Adsorption Sites as a Function of Temperature and Coverage: H2S on Si(111)-(7 × 7)

Using a variable-temperature ultrahigh vacuum scanning tunneling microscope, the authors have studied the adsorption characteristics of H{sub 2}S on Si(111)-(7 x 7). The data suggest that H{sub 2}S adsorbs dissociatively at submonolayer coverage, from 50 to 300 K, with HS bonded to an adatom and H bonded to a rest atom. The adsorption is site selective, and the adsorption site preference is temperature dependent. At 50 K, the faulted center sites are most favored for adsorption, followed y unfaulted center sites, faulted corner sites, and unfaulted corner sites. As the temperature is increased, the differences between the faulted and unfaulted halves diminish, but the center sites remain more reactive than the corner sites. At room temperature, the ratio of adsorbates on center sites versus corner sites is 2:1. The authors present an explanation to account for the non-Langmuir kintics involved in this system.

A high-speed variable-temperature ultrahigh vacuum scanning tunneling microscope

An ultrahigh vacuum scanning tunneling microscopy (STM) system capable of collecting constant-current STM images at rates exceeding 1 image/s at temperatures up to 900 K was designed, built, and tested. The microscope uses an inchworm to push a scanner assembly through a quartz tube towards the sample. When the scanner is within tunneling range of the surface, the inchworm is decoupled from the scanner, resulting in a small, rigid mechanical loop with a high resonant frequency. The cylindrical symmetry and the use of low thermal expansion materials reduces image distortion due to thermal drift. An analog proportional-integral controller with tilt correction is used to maintain a constant tunnel current. A personal computer running Microsoft WINDOWS ’95 is used to control all other instrument functions, as well as for data acquisition, manipulation, and storage. Problems associated with the non real time nature of the operating system were overcome by collecting the STM images within a virtual device driver. By taking advantage of the WINDOWS ’95 multimedia functions, the time required to display and save images was reduced below 5 ms using a standard personal computer with no special hardware other than data acquisition cards. Sample STM images of Pd(111) and halogens adsorbed on Cu(100) are presented.

New design of a variable-temperature ultrahigh vacuum scanning tunneling microscope

We present the design of a variable-temperature ultrahigh vacuum (UHV) scanning tunneling microscope which can be operated between 20 and 400 K. The microscope is mounted directly onto the heat exchanger of a He continuous flow cryostat without vibration isolation inside the UHV chamber. The coarse approach is performed with an inertial slider driven by the same piezo tube that is also used for scanning. The performance of the instrument is demonstrated by two different kinds of measurements: First we show topographic images of close packed metal surfaces with atomic resolution. Then, we present results from scanning tunneling spectroscopy measurements on WSe2 under illumination at variable temperatures.

Scanning tunneling microscopy of quasi-one-dimensional organic conductors

We have used a variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope to study the quasi-one-dimensional organic conductor, tetrathiafulvalene-tetracyanoquinodimethane. The experiment has been performed in air and UHV, at temperatures ranging from 10 to 300 K. High resolution images obtained both in air and UHV clearly revealed a one-dimensional structure. The molecular positions and orientations deduced from the real space images are in good agreement with the lattice parameters obtained from other experimental methods. Below 80 K, a commensurate 2kF charge density wave modulation in real space appeared, and its associated energy gap of about 100 mV was also observed at lower temperatures.