Session L38: Focus Session: Advances in Scanned Probe Microscopy I: Low Temperatures, Manipulation,and Optical Methods I

An ultrahigh vacuum, variable temperature scanning tunneling microscope

We will discuss the design and operation of an ultrahigh vacuum, variable temperature (2 K -- 300 K) scanning tunneling microscope (STM) system. The STM has been designed to minimize tip-sample displacements with thermal variation, allowing the tracking of single atoms over a wide range of temperatures. We will first describe STM details such as sample holder and capacitive position sensor design, as well as tip shielding to reduce scanner cross-talk. We will then discuss elements of the support system, including a low-temperature sample storage area to allow quick sample exchanges, a variable temperature cleaver and a novel counterweight system for quickly and safely lifting and lowering the experimental dewar. Finally, we will point out some common problems found in STM systems and show how we diagnosed and solved these problems.   

Scanning tunneling microscopy in high magnetic fields below 1 Kelvin

We have developed a scanning tunneling microscope (STM) which operates in a novel range of experimental parameters: ultra-high vacuum, low temperatures and high magnetic fields. Such operating conditions make the Zeeman energy for a typical magnetic system significantly larger than the thermal energy and hence one can resolve spin excitations in individual magnetic systems. In order to achieve temperatures below 4K we employ a pumped 3-He reservoir where we liquefy the 3-He with Joule Thomson expansion (without the use of a pumped 4-He reservoir). We can routinely operate the STM at 0.6K in magnetic fields up to 7T. It turned out to be surprisingly difficult to vibrationally decouple the STM from the high magnetic field, which was achieved only after investigating the low-temperature magnetic properties of all the components of the STM. This machine has been used for about 5 years to study atomic-scale magnetic systems and some examples will be discussed.   

Construction of a sub-Kelvin ultra-high vacuum scanning tunneling microscope in high magnetic field

A sub-Kelvin ultra-high vacuum (UHV) scanning tunneling microscope (STM) in high magnetic field has been built. The Besocke type scanner is mounted to the He3 pot of a bottom loading UHV compatible helium- 3 cryostat with a 9 Tesla superconducting magnet. The helium-4 reservoirs for the non-bakeable NbTi superconducting magnet and the UHV space are thermally separated in order to achieve UHV condition without overheating the magnet. A two-chamber UHV system creates reliable environment for tip and sample preparation, and surface imaging and characterization. Various atoms and molecules can be deposited at room or low temperatures. The STM system has the unique capability to probe matter at very low temperatures, in high magnetic fields, under ultrahigh vacuum conditions, and with spatial resolution below one nanometer.   

Design of a 20 mK/15 T STM system

We describe the design of a versatile ultra-high vacuum (UHV) STM system capable of ultra low temperatures ($\sim $20 mK) and high magnetic fields (15 T). A bakeable UHV dilution refrigerator (DR) was designed adopting a Joule-Thomson He3 condenser for low-noise closed-cycle operation, while maintaining the option of a traditionally pumped 1 K pot. The entire STM module can be transferred from an upper room temperature chamber, where the sample and tip are easily exchanged, into the DR in UHV. The sample holder has five isolated electrical contacts which are also used for kinematic mounting of the sample. This allows 4 probe electrical measurements to be performed simultaneously with STM measurements for microscopic transport studies. To achieve a stable environment, we use 3 stages of vibration isolation with a combination of active and passive feedback loops. Current progress will be discussed in relation to design goals.   

Atomistic constructions using a scanning tunneling microscope.

We demonstrate an atomic scale construction scheme, which is performed at an area as small as a few tens of nanometer square. In this atomic scale construction site, all the basic building blocks, single atoms, are extracted locally from the substrate using a scanning-tunneling-microscope tip. These extracted atoms are then precisely positioned on the surface to form desired structures. After the completion of the construction, the remaining debris are removed and the undesired holes near the construction site are filled with atoms/clusters to tidy up the area. This entire construction scheme closely resembles our real world construction process and can be considered as its atomic scale analog. This work is supported by NSF grant DMR-0304314 and US-DOE grant DE-FG02-02ER46012.   

Scanning Tunneling Microscope Manipulation of $\beta$-Carotene on Au(111) at 4.6 K

The properties of isolated and clustered $\beta $-carotene molecules adsorbed on a Au(111) surface were investigated using a low temperature scanning tunneling microscope (STM) at 4.6 K in an ultra-high-vacuum condition. A sub-monolayer coverage of all trans- $\beta $-carotene molecules were deposited on Au(111) via thermal evaporation using a custom-built Knudson cell. On Au(111), the $\beta $-carotene molecules can be found as a form of a cluster, as well as, isolated single molecules. Furthermore, the $\beta $-carotene molecules can have both trans and cis conformations on this surface. In order to probe the mechanical stability of the molecules and molecular clusters, we employ STM manipulation procedures. Lateral manipulations of the molecules across the surface with the STM-tip reveal that the molecules are rather stable. Furthermore, the STM manipulation experiments on $\beta $-carotene clusters often result in lateral displacement of the entire cluster indicating strong interactions between the neighboring molecules within the cluster, but a weak molecule-surface interaction. Moreover, by injecting the tunneling electrons into the molecules, the rotation of a cis $\beta $-carotene has been able to induce in a controlled manner on the surface. This work is supported by the US Department of Energy, Basic Energy Sciences grant number DE-FG02-02ER46012.   

Vertical Atom Manipulation on GaN(000$\overline 1 $) Surface at Low Temperature

We report single atom manipulation on a GaN(000$\overline 1 )$ surface at 4.6 K by using a low temperature scanning tunneling microscope (STM) tip. The nitrogen polar Ga rich GaN samples are grown on sapphire substrate by using r.f. N-plasma molecular beam epitaxy (MBE). Low temperature STM images of GaN (000$\overline 1 )$ surface reveal a novel reconstruction with a basis of 12 x 12 unit cell. For the manipulation experiment, the STM tip is first coated with Ga atom by using a controlled tip-sample contact. Using a vertical manipulation technique with the STM-tip, individual Ga atom from the tip is transferred to the GaN (000$\overline 1 )$ surface on one atom-at-a-time basis. The successful atom deposition is conformed by subsequent STM imaging. Here, the controlled STM tip-sample contact plays a crucial role in an atom deposition process. This procedure allows construction of nanostructures on a MBE grown semiconductor surface with atomic scale precision. This work is financially supported by a NSF-NIRT grant no. DMR-0304314.   

Design and Construction of a UHV-LT-STM for Tip-Enhanced Optics.

The combination of optical techniques and scanning tunneling microscopy (STM) provides insight into a diverse set of physical processes including surface chemistry, surface-photon interactions, and spin scattering in semiconductors. We have designed and built a novel, cryogenic temperature, ultrahigh vacuum STM which utilizes a maneuverable high numeric aperture lens in proximity to the tunnel junction. The microscope currently operates at a base temperature of 12.5 K with 10 pm tip stability. Our initial efforts are focused on studies of photo-chemical reactions and chemical identification by tip-enhanced Raman spectroscopy (TERS). Chemically-etched Ag tips are optimized for field enhancement with characterization by scanning electron microscopy and collection of the plasmon emission from the tip. The optical setup for TERS has been tested utilizing the surface-enhanced Raman signal from the laser dye R6G. The field enhancement of metallic nanostructures can be tuned with atomic manipulation for single molecule spectroscopy and near-field microscopy. http://www.physics.ohio-state.edu/$\sim $jgupta   

Stress Imaging in Indented Si Wafers by Confocal Raman Microscopy

Controlling stress and strain, and consequently, carrier mobility in semiconductor devices is one of the main goals of recent electronic industry. On the other hand, fracture propagation is commonly related to performance degradation in microelectronic and microelectromechanical (MEMS) devices. As miniaturization reaches submicron scales, characterization tools with improved resolution and capable to detect buried surfaces is required. In this work we present confocal Raman imaging in Si wafers to analyze stress and fracture by means of hyperspectral measurements (typically 128x128 spectra). We analyzed indented Si wafers presenting wide range of plastic deformation and fractures. Wide scans (up to 150x150 $\mu $m$^{2})$ as well as high-resolution scans depict the stress distribution around indented regions and side fractures. Some of the samples were covered with 8 nm of Ti deposited in LN$_{2}$ temperature. In these samples we acquired hyperspectral images in subsurface conditions and detected possible influences of thermal budged in the stress distribution. We also demonstrate depth sensitivity in a vertical scan. Images suggest 0.3 $\mu $m resolution.   

Tip Enhanced Raman Scattering of Strained Silicon with Single and Multiple Probe Scanned Probe Microscopes.

Raman spectroscopy is an effective tool for the identification and analysis of molecular components of complex materials. The spatial resolution of Raman spectroscopy is limited by the wavelength of the light. One approach to overcome this drawback is Surface Enhanced Raman Scattering (SERS). This technique uses nanometric interactions between metal structures and surfaces to effect enhancement of the Raman signals. An important mechanism for enhancement originates from an electrostatic lightning rod effect due to the excitation of localized surface plasmon resonances. This is accomplished in a scanned probe microscopy context by employing an ultra-sharp metalized tip that is brought into a focused laser spot on the sample surface thereby enhancing the Raman signal. In this technique also known as Tip Enhanced Raman Scattering (TERS) the electrical field is locally enhanced near the sharp metalized tip. Rastering the sample should then allow for Raman imaging with nanometric resolution. Within this context it will be shown that multiple probe scanned probe microscopes have considerable potential in such tip enhanced applications.   

High Efficiency Surface Plasmon Enhanced Near-field Scanning Optical Microscope Probe Development.

We present results from the development of novel, high throughput, near-field scanning optical microscope (NSOM) probes based on excitation of surface plasmons. The probe consists of an opaque noble metal film with a bullseye grating cavity on the input surface, and a sharp metal post on the output surface. The post is centered inside the inner grating ring and surrounded by a sub-wavelength ring aperture. The grating structure couples incident photons into surface plasmon waves. The transmission efficiency is enhanced for wavelengths where the plasmon is resonant with the cavity. Topographic and optical resolutions are determined by the sharpness of the metal post. This design is anticipated to provide the high spatial resolution of an apertureless NSOM combined with the experimental convenience of an aperture NSOM. Experimental and computational results from test structures will be presented. This material is based on work supported by the National Science Foundation under Grant No. DMI-0522281.   

A Silicon MEMS Probe Integrated with Light Emitting Nanoparticles on Tip for Near-field Scanning Optical Microscopy

We have built a nanoscale light emitting diode (LED) on a silicon MEMS probe for near-field scanning optical microsopy (NSOM). The LED was made of semiconductor nanoparticles electrostatically trapped between a pair of silicon electrodes located on the tip. The probe was microfabricated on a Silicon-on-Insulator (SOI) wafer. The facing electrodes were made by cutting a lithographically patterned device layer using a focused ion beam (FIB). When the voltage was applied, the nanoparticles were polarized and attracted to the gap along the electric field gradient. Basic parameters of a nanoparticle-trapped LED were measured. The probe was attached to a tuning fork and mechanically oscillated. The resonant frequency of the tuning fork was originally about 100KHz and was dampened to 93.0KHz with the probe attached. As the tip approaches the surface of the sample, a drag force acting on the tip changes the oscillating amplitude; measured as a voltage signal from the fork, which in feedback allows the tip to be positioned in the near-field, roughly 5-10nm from the surface. Successful fabrication of the light emitting NSOM probe leads to integrated ``light-source free'' optical scanning arrays suitable for novel applications in nanomaterial characterization and biology.   

Spectroscopic near-field microscopy using frequency combs in the mid-infrared

\newcommand{\wn}{cm\textsuperscript{-1}} We introduce a new concept of spectroscopic scattering-type near- field optical microscopy that records 200 \wn \ broad infrared spectra at each pixel during scanning. Two coherent beams with harmonic frequency-comb spectra are employed, one for illuminating the scanning tip, the other as reference for multi- heterodyne detection of the scattered light. Our implementation yields amplitude and phase spectra centered at 950 \wn (this band can be tuned between 700 and 1400 \wn). A new technique of background suppression is introduced which is enabled by the short, 10 $\mu$s ``snapshot'' acquisition of infrared spectra which allows time-resolving the tapping motion. Thus we demonstrate broad-band mid-infrared near-field imaging that is essentially free of background artefacts.\\ (1) A. Schliesser, M. Brehm, F. Keilmann \& D. W. van der Weide \emph{Frequency-comb infrared spectrometer for rapid, remote chemical sensing} Optics Express, 13, 9029-9038 (2005)\\ (2) M. Brehm, A. Schliesser \& F. Keilmann \emph{Spectroscopic near-field microscopy using frequency combs in the mid-infrared} Optics Express, 14 ,11222-11233 (2006)   

Element specific imaging by STM combined with synchrotron radiation light

Atomically resolved imaging with a capability of elemental identification is one of the ultimate goals in the development of microscopy. Using scanning tunneling microscopy (STM), which provides us atomically resolved surface images, many attempts have been performed for elementally contrasted images. However, since STM basically probes electronic states near the Fermi energy, it is difficult to obtain definite ``fingerprints'' of elements. Here, we report on a new method to obtain elemental information by STM combined with synchrotron radiation light. We found that, by exciting core electrons with the soft-X-ray irradiation and detecting emitted electrons with the STM probe tip, we can obtain X-ray absorption spectra bearing elemental information of the sample. From the photo-induced current measured during the tip scanning over the surface, element specific images were obtained. An estimated spatial resolution of the chemical imaging is less than 20 nm, better than that achieved by photoemission electron microscopy.   

Waveguide Characterization Using Shear Force Scanning Optical Microscopy.

Waveguide characterization is an essential task in the development of photonic integrated circuits for a variety of applications, including biosensors and next generation optical interconnects. A shear force SOM (scanning optical microscope) is being developed for characterizing waveguide evanescent fields as well as scattered light in both the near field and the far field. These methods correspond to photon scanning tunneling microscopy, proximity scanning optical microscopy, and scatter imaging, respectively. Shear force feedback eliminates noise due to scattered light introduced by a second light source required for conventional optical feedback systems based on reflective cantilevers. Additionally, the shear force feedback configuration simplifies raster scanning of the probe rather than the sample allowing easier coupling to multiple waveguides on the sample. The distribution of scattered light intensity can be correlated with features in the evanescent fields that may prove useful for waveguide sensors.