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1. Development of a scanning tunneling microscope for the carrier profiling of semiconductors by scanning frequency comb microscopy

   https://doi.org/10.1109/WMED.2017.7916926  

   Spencer, Greg ; Hagmann, Mark J. ( April 2017 , Development of a scanning tunneling microscope for the carrier profiling of semiconductors by scanning frequency comb microscopy)

   Summary form only given. We are developing a scanning tunneling microscope that is portable and optimized for scanning frequency comb microscopy (SFCM) as one part of our effort to complete a prototype for the carrier profiling of semiconductors by SFCM. Conventional integral or integral plus proportion feedback control of the tunneling current in a scanning tunneling microscope (STM) is satisfactory once tunneling has been established but may cause tip-crash by integral windup during coarse approach. In tip-sample contact images with atomic-resolution may be obtained but the microwave frequency comb ceases because there is no optical rectification and scanning tunneling spectroscopymore »also fails. We are studying a new control algorithm based on approximating the tunneling current as a polynomial in the bias voltage where the coefficients in this polynomial are not required. It is noted that hanges in the apparatus, as well as the algorithms used for feedback control in the STM, are required to optimize this instrument for measuring the microwave frequency comb.« less
2. Ultra-sharp and surfactant-free silver nanowire for scanning tunneling microscopy and tip-enhanced Raman spectroscopy

   https://doi.org/10.1039/C8NR08983C  

   Liu, Qiushi ; Kim, Sanggon ; Ma, Xuezhi ; Yu, Ning ; Zhu, Yangzhi ; Deng, Siyu ; Yan, Ruoxue ; Zhao, Huijuan ; Liu, Ming ( April 2019 , Nanoscale)

   Chemically-synthesized single-crystalline silver nanowire (AgNW) probes can combine the scanning tunneling microscopy (STM) technique with tip-enhanced Raman scattering spectroscopy (TERS) for complementary morphological and chemical information with nanoscale spatial resolution. However, its performance has been limited by the blunt nanowire tip geometry, the insulating surfactant layer coating AgNW surfaces, and the thermal-induced mechanical vibrations. Here, we report a reproducible fabrication method for the preparation of sharp-tip AgNW-based TERS probes. By removing the polyvinylpyrrolidone (PVP) surfactant molecules from the AgNW surfaces for stable electrical conductivity and controlling the protruding length with μm-level accuracy for improved mechanical stability, we demonstrate atomic-resolution STMmore »imaging with the sharp-tip AgNW probe. Furthermore, the sharp-tip AgNW has an excellent TER enhancement (∼1.1 × 10 6 ), which is about 66 folds of that achieved by regular AgNWs. Our experiments demonstrate that AgNWs with clean interfaces and the proper tip geometry can provide reliable and reproducible STM and TER characterizations, which remove the hurdles preventing the implementation of AgNW in STM-based near-field optical applications for a broad community.« less
3. Electrode control methodology for a scanning tunneling microscope

   Hagmann, M. ( February 2018 , U.S. patentSearch claims & abstracts)

   A control methodology for scanning tunneling microscopy is disclosed. Instead of utilizing Integral-based control systems, the methodology utilizes a dual-control algorithm to direct relative advancement of a STM tip towards a sample. A piezo actuator and stepper motor advances an STM tip towards a sample at a given distance until measuring a current greater than or equal to a desired setpoint current. Readings of the contemporaneous step are analyzed to direct the system to change continue or change direction and also determine the size of each step. In simulations where Proportion and/or Integral control methodology was added to the algorithmmore »the stability of the feedback control is decreased. The present methodology accounts for temperature variances in the environment and also appears to clean and protect the tip electrode, prolonging its useful life.« less
4. Investigation of Protein/Lipid Interactions via Scanning Probe Acceleration Microscopy: Theory and Experiment

   https://doi.org/10.1115/DETC2012-70228  

   Legleiter, Justin ; Burke, Kathleen A. ; Yates, Elizabeth A. ( August 2012 , ASME 2012 International Design Engineering Technical Conferences and Computers and Information in Engineering Conference)

   There is great interest in the application of proximal probe techniques to simultaneously image and measure mechancial properties of surfaces with nanoscale spatial resolution. There have been several innovations in generating time-resolved force interaction between the tip and surface while acquiring a tapping mode AFM image. These tip/sample forces contain information regarding mechanical properties of surfaces in an analogous fashion to a force curve experiment. Here, we demonstrate, via simulation, that the maximum and minimum tapping forces change with respect to the Young’s modulus and adhesiveness of a surface, but the roughness of the surfaces has no effect on themore »tapping forces. Using these changes in tapping forces, we determine the mechanical changes of a lipid membrane after exposure to a huntingtin exon1 (htt exon1) protein with an expanded polyglutamine (polyQ) domain. Expanded polyQ domains in htt is associated with Huntington’s disease, a genetic neurodegenerative disorder. The htt exon1 protein caused regions of increased surface roughness to appear in the lipid membrane, and these areas were associated with decreased elasticity and adhesion to the AFM probe.« less
5. Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM)

   https://doi.org/10.1109/WMED.2017.7916928  

   Birch, Timothy ; Hagmann, Mark J. ( April 2017 , Increased versatility for carrier profiling of semiconductors by scanning frequency comb microscopy (SFCM))

   We are developing a new method for the carrier profiling of semiconductors that shows promise for nm-resolution which is required at the new sub-10 nm lithography nodes. A modelocked ultrafast laser focused on the tunneling junction of a scanning tunneling microscope (STM) generates a regular sequence of pulses of minority carriers in the semiconductor. Each pulse of carriers has a width equal to the laser pulse width (e.g. 15 fs). In the frequency domain, this is a microwave frequency comb (MFC) with hundreds of measurable harmonics at integer multiples of the laser pulse repetition frequency (e.g. 74 MHz). After themore »minority carriers diverge rapidly into the semiconductor as a Coulomb explosion, the pulses become broader and decay, so that the MFC has less power with a spectrum limited to the first few harmonics. The frequency-dependent attenuation of the MFC is determined by the resistivity of the semiconductor at the tunneling junction so SFCM is closely related to Scanning Spreading Resistance Microscopy (SSRM). Harmonics of the MFC are measured with high speed, and high accuracy because the signal-to-noise ratio is approximately 25 dB due to their extremely narrow (sub-Hz) linewidth. Now we superimpose a low-frequency signal (e.g. 10 Hz) on either the applied bias or the voltage that is applied to the piezoelectric actuators of the STM to cause sidebands at each harmonic of the MFC which are less affected by the artifacts.« less