AVS1996 Session NS-MoM: Novel Measurements and Advances in Scanned Probe Microscopy

Monday, October 14, 1996 8:20 AM in Room 202 A/B

Monday Morning

Start	Invited?	Item
8:20 AM		NS-MoM-1 Scanning Thermal Microscopy at Sub-10 nm Spatial Resolution K. Luo, Z. Shi, J. Varesi, A. Majumdar (University of California, Santa Barbara) Scanning thermal microscopy (SThM) has the unique capability of using a single probe to study power dissipation in novel devices, to conduct photothermal imaging and spectroscopy, for sub-surface imaging and, in general, to investigate the energetics and thermodynamics of a variety of physical phenomena. In this paper, we will present SThM of novel nanodevices, such as a vertical cavity quantum-well laser and magnetoresistive reading heads, at sub-10 nm spatial resolution. In addition, nanometer-scale sub-surface imaging will be presented. The key to the high resolution lies in the probe design. A new technique of nanofabricating a temperature sensor at the very tip of a standard silicon nitride cantilever AFM probe will be described. This technique relies on opening a 100 nm diameter hole in a metal film, by field-induced evaporation arising from voltage pulses. The paper will also discuss the mechanisms of tip-sample energy flow, its interactions with surface morphology, as well as the reasons for sub-10 nm spatial resolution. The prospects of improving the resolution to sub-1 nm will also be explored.
8:40 AM		NS-MoM-2 Infrared Detection and Imaging using Microcantilevers T. Thundat, P. Oden, P. Datskos, R. Warmack (Oak Ridge National Laboratory) The feasibility of micromechanical infrared (IR) detection and imaging using microcantilevers is demonstrated. Microcantilevers provide a simple means for developing single- and multi-element sensors for infrared radiation that are smaller, more sensitive and lower in cost than quantum or thermal detectors. Microcantilevers coated with a heat absorbing layer undergo bending due to the differential stress originating from the bimetallic effect. Bending is proportional to the amount of heat absorbed and can be detected using optical or electrical methods such as resistance changes in piezoresistive cantilevers. A noise equivalent power (at a modulation frequency of 30 Hz) was found to be 3.5 nW/\srHz\. Thermal imaging was achieved by scanning the infrared image of the object formed at the focal plane of an infrared collecting mirror using a single thermally sensitive cantilever. We will present the IR images of objects obtained with this microcantilever infrared detector and address the issue of sensitivity and speed of image acquisition.
9:00 AM	Invited	NS-MoM-3 Thermovoltage in Scanning Tunneling Microscopy D. Hoffmann, J. Grand, T. Kunstmann, A. Haas, M. Dietsche, R. M\um o\ller (Universit\um a\t Stuttgart, Germany) The thermovoltage which will be discussed arises if the tunneling tip and the sample are at different temperatures. For zero external bias voltage the difference of the Fermi distribution for both electrodes will lead to a thermally driven current since in general the balance between forward and backward tunneling of electrons will be disturbed. If the external current is zero a bias voltage is found leading to a tunneling current of same magnitude but opposite direction. This voltage will be referred to as thermovoltage. It provides an analytical tool with excellent lateral resolution which is very sensitive to minor variations of the local electronic density of states of tip and sample in the vicinity of the Fermi level. The investigation of the (111) surfaces of the noble metals reveals several details of the electronic structure. At the edges of monoatomic steps the modification of the projected bulk states yields a very localized variation of the thermovoltages. For Cu(111) interference patterns of electronic surface states become visible. Furthermore Ag(111) shows a local variation with atomic periodicity. For the first time, this provides evidence that the energy dependence of the electronic density of states varies between a position right above an atom or in between atoms of a metallic surface. The thermovoltage may also be used for the investigation of chemically heterogeneous surfaces. The growth of Cu on Ag(111), Fe on Ag(111) and Cu(111) has been analyzed. These examples demonstrate that the measurement of the thermovoltage should be especially useful for the study of alloying processes. If the adsorption of molecules on metallic surfaces is investigated additional information can be obtained, as will be shown for a monolayer of copper phthalocyanine on Ag(111).
9:40 AM		NS-MoM-5 Functionalization of STM Tips with C\sub 60\ K. Kelly, D. Sarkar, S. Oldenburg, G. Hale, N. Halas (Rice University) We have succeeded in functionalizing scanning tunneling microscope (STM) tips with C\sub 60\ molecules. Adsorption of fullerene molecules onto the STM tip at the tunneling junction is characterized by an "inverse" imaging technique based on scanning tunneling microscopy.(1) This imaging technique permits a determination of the number of molecules on the STM tip in the tunneling junction region, their relative placement and also their orientation under certain conditions. Tips prepared with a single fullerene adsorbate at the tunneling junction yield excellent atomic resolution of graphite surfaces,(2) indicating that the adsorbate molecule functions as a true tunneling site on the STM tip. These STM tips have permitted us to image electron scattering effects near defects on graphite surfaces that have been previously predicted (3) but have heretofore not been observable with standard metallic tips. Overlap of the C\sub 60\ LUMO level with the Fermi energy of the tip yields a narrow, partially filled adsorbate level that facilitates the imaging of these electron scattering effects. These results demonstrate that functionalizing a STM tip with anadsorbate of an appropriate electronic structure may allow greater control of the electronic interactions between the STM tip and sample.(1) K. F. Kelly et al., J. Vac. Sci. Technol. B 14, in press.(2) J. Resh et al., Surf. Sci. 316, L1061 (1994).(3) H. A. Mizes and J. S. Foster, Science 244, 599 (1989).
10:00 AM		NS-MoM-6 Distinguish between Adatoms in Faulted and Unfaulted Halves of Si (111) 7x7 by Noncontact Atomic Force Microscopy M. Suzuki, K. Okiguchi, H. Sugimura, N. Nakagiri (Nikon Co., Japan) Very recently, several demonstrations have been made to indicate that the noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum has an atomic resolution. Obtained images of Si(111)7x7 or InP(110) show not only periodic lattice structures but also atomic defects, which is the evidence of the true atomic-scale resolution. However, the imaging mechanism of NC-AFM has not studied throuthly, especially dependencies of the oscillation amplitude of the cantilever and the setting of the frequency shift. We have studied the oscillation amplitude dependence of NC-AFM images of Si(111)7x7. The NC-AFM used was the AutoProbe VP (Park Scientific Instruments). This apparatus uses the so-called piezolever and the frequency modulation method. This cantilever has a resonant frequency of 120 kHz, a force constant of 20 N/m, and a Q value of 24000 in vacuum. At a fixed frequency shift, NC-AFM images were acquired at various amplitudes. At less than an amplitude of 30 nm, no atomic image was observed. At an amplitude of 35 nm, an obtained clearly-resolved atomic image showed bright 6 atoms in a half of the 7x7 unit cell and less bright 6 atoms in the other half of the unit cell. These two halves correspond to faulted and unfalted halves of the unit cell: the unit cell consists of faulted and unfalted halves according to the DAS model. We can not tell which is which. However, this experimental result demonstrates that NC-AFM can detect the faint difference of interactions between a very top surface atom and other surrounding atoms of the sample, in other word, site difference. At an amplitude of 40 nm, such difference was not observed.
10:20 AM		NS-MoM-7 Non-Optical Feedback in Scanning-Probe Microscopes using a Piezo Tuning Fork as a Sensor H. Edwards, W. Duncan (Texas Instruments, Inc.) In cantilever-based scanning-probe microscopy (SPM) as well as near-field scanning optical microscopes (NSOM), tip-sample distance regulation is achieved using a laser beam bounced off of the cantilever or the tip-sample junction, respectively. While laser-beam-bounce techniques are well developed and stable for SPM and NSOM, there are some applications where it is desirable to control the light in the sampling volume at the tip-sample junction. For imaging and spectroscopic applications of NSOM, the optical properties of the sample are of interest and hence the external light introduced by the feedback laser is unwanted. For cantilever-based SPM, electrical measurements of semiconductors require that the illumination be controlled. In this paper, we discuss the exploration of a new non-optical feedback method proposed by Khaled Karrai and Robert D. Grober, Appl. Phys. Lett. 66, 1842 (1995). In this method, the probe is attached to a tine of a miniature quartz tuning fork. The tuning fork is driven with a small-amplitude signal near its mechanical resonant frequency to drive the tip. The impedance of the tuning fork is monitored as J. W. P. Hsu, Mark Lee, and B. S. Deaver, Rev. Sci. Inst. 66, 3177 (1995) proposed for a different sensor configuration. By measuring the AC current component in a voltage-driven tuning fork, the impedance is measured and stable tip-sample feedback may be attained with as little as an Angstrom of dither amplitude. Using a 0.001 inch diameter W wire mounted on the end of a tuning fork, we have obtained reproducible SPM images of the surface micro-texture of a polished Si wafer with sub-Angstrom RMS roughness. We will also present other results obtained in the meantime.
10:40 AM		NS-MoM-8 Molecular and Nanoscale Electronic Properties of Organic Thin Films L. Bumm, J. Arnold, M. Cygan, D. Allara (Pennsylvania State University); J. Tour (University of South Carolina); P. Weiss (Pennsylvania State University) The molecular and nanoscale electronic structure of organic thin films is studied using scanning tunneling microscopy (STM). Thiols adsorbed onto Au(111) are an ideal model system for studying molecular and nanoscale electronic structure. We use self-assembled monolayers of alkanethiolates as a matrix to dilute and to isolate "guest" molecules in the film. STM can distinguish the isolated guest molecules from the alkanethiolate matrix even for guest-matrix differences as slight as one methylene unit in alkyl chain length. The microwave frequency alternating current STM (ACSTM) provides additional contrast which complements conventional STM topographic images. The combination of STM and ACSTM are used to differentiate between molecular wire candidates and the surrounding alkanethiolate matrix. These techniques allow us to measure the electronic properties of single molecules isolated by the matrix.
11:00 AM		NS-MoM-9 Probing the Nano-defect Induced CDW Metastable States by Low-T UHV STM J. Zhang, J. Huang, J. Liu, C. Lieber (Harvard University) A low-temperature, ultrahigh vacuum STM has been used to create and study nanoscale defects in layered transition metal dichalcogenide materials that exhibit charge density wave (CDW) states. These studies are motivated by the recognition that nanometer scale defects might serve as points with which to probe fundamental properties of correlated electron system such as scattering and pinning. In our experiments, we used the STM tip to make controllably nanometer size defect (nD) on single crystal surface of metal dichalcogenide materials that exhibit CDW states. With STM and STS, we subsequently probed both the structure and electronic properties of the CDW around the nDs. The common theme from our studies of different materials is that the CDW state is trapped in a metastable state locally around the nD at 4.2 K. In both 1T-TaSe2 and 1T-TaS2 samples, we trapped the CDW state in a locally discommensurate state by nDs. In 1T-TaSe2 sample, this discommensurate state manifests itself as individual discommensuration lines end in atomic dislocation cores. The interaction between nearby nDs of different symmetry has also been studied. In 1T-TaS2, however, an irregular domain structure separated by the sharp discommensuration lines was observed. In other crystal forms of the TaX2 materials, a more striking effect of the nDs has been observed. Specifically, we identified conditions for effecting locally a solid -solid phase transition. The implications of these results will be discussed.
11:20 AM		NS-MoM-10 Surface Science at LHe Temperatures with a New Dedicated Low Temperature UHV STM System T. Becker (Omicron Vakuumphysik GmbH, Germany); H. Hoevel, M. Tschudy, B. Reihl (IBM, Rueschlikon, Switzerland) A new dedicated Low Temperature UHV STM for investigations of physical properties at low temperatures such as quantum effects, single electron tunnelling, and charge density waves as well as surface modifications and spectroscopy is presented. In order to achieve sample temperatures as low as 5 K, and to ensure a high thermal stability the STM head uses a concentric system of bath cryostats. Openings providing optical access for optical measurements even at low temperatures are shielded with IR-blocking filters, and can be closed completely by a door mechanism. Efficient vibration isolation is realised using spring suspension with eddy current damping. At 5 K the single tube scanner offers a scan range of approx. 2 x 2 um. The LT STM contains a 3-dimensional,piezoelectrically driven coarse positioning unit for STM tip positioning, and tip and sample exchange can both be operated at low temperature. Atomically resolved images of Au(111) at 5 K in UHV demonstrate the excellent performance of the LT STM; hexagonal ring structures have been observed, which can be attributed to variations of the local electronic structure caused by single adsorbate species. As a typical application example the appearance of charge density waves (CDW) on NbSe2 was studied. CDW on NbSe2 are known to only occur below a transition temperature of 33.5 K. Different domains of the incommensurate CDW, and the typical wave length of about 3a could clearly be identified. Low temperature I/V spectroscopy measurements have been performed on Au-clusters on graphite.
11:40 AM		NS-MoM-11 A Novel UHV SPM Capable of Three-Axis Movement, Variable Temperature Imaging and MFM J. Liu (Park Scientific Instruments); J. Patron (FSI International); K. Mike, N. Thai, L. Fred, S. Presley (Park Scientific Instruments) We have developed a novel ultrahigh vacuum, variable temperature scanning probe microscope. The instrument utilizes our proprietary technique, piezoresistive cantilevers for the contact and noncontact AFM, and allows interchangeable STM, AFM operations in situ. The instrument allows both STM and AFM imaging at elevated temperature. Preliminary AFM images obtained at elevated temperature will be presented. The microscope features a high resolution, three-axis, inertial-driven translation stage with an open access for customize-designed sample holder With magnetically coated pizeoresistive cantilevers, we have carried out MFM experiments successfully in situ and will present some data.