Session J1: Focus Session: Advances in Scanned Probe Microscopy II: High Frequencies & Optical Techniques

Towards Magnetic Resonance Imaging of Semiconducting and Biological Nanostructures

In recent years a technique combining nuclear magnetic resonance (NMR) and sensitive force microscopy has emerged as a viable method for doing magnetic resonance imaging (MRI) on the nanometer scale [1]. This method, known as magnetic resonance force microscopy (MRFM), has the potential to create three-dimensional (3D), non-destructive, sub-surface images of the density of particular nuclear magnetic moments with isotopic contrast. Resolution better than $10\,nm$ has been achieved with ${ }^1\mbox{H}$ in a single virus particle [2]. Here we discuss the application of this technique to nanobiological samples, such as viruses, small bacteria, or cell membranes, and to various semiconductor nanostructures including quantum wells (QWs) and nanowires (NWs). In particular, we focus on the sample preparation challenges presented by these samples. Transfer and attachment of these sub-micrometer samples to our micrometer-sized force sensor includes the use of a focused ion beam (FIB) technique and manual micromanipulators used together with optical microscopy.\\[4pt] [1] Nanotechnology 21, 342001 (2010). \newline [2] Proc. Nat. Acad. Sci. U.S.A. 106, 1313 (2009).   

Nanoscale MRI of electron spins at millikelvin temperatures

Magnetic Resonance Imaging by Atomic Force Microscopy (MRI-AFM) combines the non-destructive subsurface sensitivity of an MRI scanner with the high spatial resolution of an Atomic Force Microscope. It is a powerful technique to detect a small number of spins that relies on force detection by an ultrasoft, magnetically tipped cantilever and selective magnetic resonance manipulation of the spins. In order to minimize the thermomechanical noise of the cantilever and to increase spin polarization, MRI-AFM should be carried out at ultralow temperatures. Therefore, we developed a SQUID-based detection technique, which avoids heating of the cantilever and the spin sample. Using this technique, we demonstrate the manipulation and detection of dangling bond paramagnetic centers on a silicon surface at temperatures as low as 30 millikelvin. The fluctuations of these unpaired electron spins are supposedly linked to $1/f$ magnetic noise and decoherence in SQUIDs as well as in several superconducting and single spin qubits. We find evidence that spin diffusion plays a key role in the dynamics of spins at low temperatures.   

ESR induced anomalous polarization in Magnetic Resonance Force Microscopy

Mechanically detecting electron spin resonance has opened up new avenues of performing magnetic resonance detection and imaging to an individual spin-labeled macromolecule. The large gradient field from the magnetic tipped cantilever creates selective resonance conditions for each spin label in the macromolecule. The detection is made through the shifts in the cantilever self-oscillating frequency due to the back action on to the cantilever from the resonating spin polarization. In order to improving the detection sensitivity, great efforts have been made to transfer polarization of electron spins to nearby nuclear spins. Here, we reported an anomalous frequency shift in our mechanically detected ESR experiment. This ESR induced anomalous frequency shift, however is larger in amplitude and slower in relaxation time than ESR frequency shift. We will discuss that this anomalous polarization are potentially due to the dynamic nuclear polarization (DNP) mechanism.   

Nanomechanical detection of nuclear magnetic resonance using a silicon nanowire oscillator

``Bottom-up'' nanomechanical devices such as nanowires, nanotubes, and grapheme oscillators have previously been proposed as next-generation scanning probe force and mass sensors because of their potential for ultralow mechanical dissipation. Here, we report the use of a radio frequency silicon nanowire mechanical oscillator as a nuclear magnetic resonance force sensor to detect the statistical polarization of $^{1}$H spins in polystyrene. Using a specialized scanning probe microscope, as well as a polarization-enhanced fiber-optic interferometer, we operate the nanowire as a force sensor at cryogenic temperatures. Nanowires of the type we study have very low intrinsic dissipation, and they experience negligible increase in dissipation as close as 10 nm from a surface. In order to couple the $^{1}$H spins to the nanowire oscillator, we have developed a magnetic resonance force detection protocol which utilizes a nanoscale current-carrying wire to produce large time-dependent magnetic field gradients as well as the rf magnetic field. Under operating conditions, the nanowire exhibited an ultralow force noise of 1.9~aN$^{2}$/Hz in the measurement quadrature. Further progress toward nanometer scale magnetic resonance imaging using this technique is discussed.   

Mechanism for Edge-enhanced Optical Response of Tip Induced Plasmonic Emission

In the investigation of tip induced plasmonic emission supplemented by STM, molecular layers are always viewed as spacers suppressing the induced emission. However, our nanoscale photo mapping of H$_{2}$TBPP/Ag(111) strikingly showed enhanced emission at the molecule-island edge, which is even remarkable higher than that on the bare Ag surface. To understand this intriguing phenomenon, DFT calculations have been carried out. We found that when an organic molecule is absorbed on Ag surface, its HOMO couples with the Ag states around the Fermi level, and forms an inelastic tunneling channel. The involved states of this channel accumulate in the interface and around the phenyl groups. Therefore, when the STM tip locates at the edge of the molecular island, the proportion of the inelastic tunneling current increases and the photo emission is enhanced. We also found that this edge-enhanced photo response is generic; the investigations of other molecule/metal systems demonstrate similar results.   

Adiabatic Plasmon Nanofocusing for Ultrashort Pulses and Spectroscopy

The simultaneous control of optical fields on both nanometer spatial and femtosecond time scales would enable direct spectroscopic access to the elementary electronic and vibrational excitations in matter. Here, we utilize adiabatic surface plasmon polariton (SPP) nanofocusing on free-standing 3D tapered metal tips in order to generate nanometer confined field localization at the tip apex. Using the second harmonic generation (SHG) at the tip apex we perform MIIPS pulse optimization and frequency-resolved optical gating (FROG) characterization of the nanofocused pulses. With the combination of high bandwidth coupling using a chirped grating, pulse-shaping, and low-dispersion nanofocusing, we can achieve full optical control on the nanoscale, from $<$ 16 fs pulse duration to arbitrary optical waveforms. This technique enables linear and non-linear plasmon-enhanced spectroscopy, with the simultaneous temporal control over ultrashort pulses opening the possibility for true time-resolved scanning-probe imaging. We demonstrate this capability for background-free probing of individual molecular and nanocrystalline systems.   

Near-Field Enhanced UV Resonance Raman Spectroscopy Using an Aluminum Bow-tie Nano-antenna

An aluminum bow-tie nano-antenna is combined with the resonance Raman effect in the deep UV to dramatically increase the sensitivity of Raman spectra to a small volume of material, such as benzene used here. By carefully choosing the right geometric parameters for the nano-antenna, we achieved a gain of a half million in signal intensity from the near field enhancement due to the surface plasmon resonance in the aluminum nanostructure. The on-line resonance enhancement contributes another factor of several thousands, limited by the laser line width. Thus, an overall gain of billions is achieved. We also demonstrated that the strong electric field gradients inside the bow-tie gap induce gradient-field Raman peaks for several strong IR modes.   

Multimodal Imaging of Heterogeneous Materials

New materials for highly specified applicaltions can either have mono-atomic flat surfaces or a roughness of several hundred micro- or millimeter. In the past two decades, AFM was the main techniques used to characterize the morphology of nano-materials. On the other hand, Raman spectroscopy is known to be used to unequivocally determine the chemical composition of a material. By combining Raman spectroscopy with high resolution confocal optical microscopy, the analyzed material volume can be reduced below 0.02 $\mu $m$^{3}$, thus leading to the ability to acquire Raman images with diffraction limited resolution. The combination of confocal Raman microscopy with Atomic Force Microscopy (AFM) is a breakthrough in microscopy. Using such a combination, the high spatial and topographical resolution obtained with an AFM can be directly linked to the chemical information provided by confocal Raman microscopy. True Surface Microscopy, allows confocal Raman imaging guided by the surface topography obtained by an integrated non-contact optical profilometer. Large-area topographic coordinates from the chromatic confocal profilometer can be precisely correlated with the large area confocal Raman imaging data. This allows true surface Raman imaging on heavily inclined or rough surfaces, with the sample surface held in constant focus, while maintaining highest confocality. In summary, the combination of confocal Raman microscopy with AFM and true surface microscopy allows the characterization of materials at high, submicron resolution, as well as on mm-rough surfaces across large areas.   

Local 2D-2D tunneling in high mobility electron systems

Many scanning probe techniques have been utilized in recent years to measure local properties of high mobility two-dimensional (2D) electron systems in GaAs. However, most techniques lack the ability to tunnel into the buried 2D system and measure local spectroscopic information. We report scanning gate measurements on a bilayer GaAs/AlGaAs heterostructure that allows for a local modulation of tunneling between two 2D electron layers. We call this technique Virtual Scanning Tunneling Microscopy (VSTM) [1,2] as the influence of the scanning gate is analogous to an STM tip, except at a GaAs/AlGaAs interface instead of a surface. We will discuss the spectroscopic capabilities of the technique, and show preliminary results of measurements on a high mobility 2D electron system.\newline [1] A. Sciambi, M. Pelliccione \textit{et al.}, Appl. Phys. Lett. \textbf{97}, 132103 (2010).\newline [2] A. Sciambi, M. Pelliccione \textit{et al.}, Phys. Rev. B \textbf{84}, 085301 (2011).   

Infrared Phonon Fingerprinting of Nanocrystals through Broadband Near-Field Spectroscopy

Near-field infrared spectroscopy has recently been demonstrated with the capability to resolve optical properties of sub-wavelength sample areas across a broad range of infrared frequencies. This method holds promise for the direct identification of sub-wavelength chemical composition in nanostructured and heterogeneous samples. We apply this technique to the study of phonon-resonant silicon carbide nanocrystals tens of nanometers in size using an apertureless scanning near-field optical microscope (SNOM) coupled to a pulsed broadband infrared laser source and FTIR spectrometer. We present measurements of nanocrystal near-field spectra in the range of 700-1200 cm$^{-1}$ evaluated in comparison with the near-field spectra of bulk silicon carbide, calibrated using ellipsometry. A detailed analytic model of the probe-sample near-field interaction is applied for the identification of nanoscale resonant size effects. These techniques provide a powerful method for identifying and characterizing sub-wavelength nanocrystals in heterogeneous samples via near-field infrared ``phonon fingerprinting.''   

Creation of Electron Trap States in Silicon Dioxide By Local Electron Injection

Over a decade ago, the Scanning Tunneling Microscope was shown capable of desorbing single hydrogen atoms from the surface of hydrogen terminated silicon.\footnote{T.C. Shen et. al. Science 268, 1590 (1995).} The resultant dangling bonds can act as atomic scale quantum dots.\footnote{M. B Haider et. al. PRL 102, 046805 (2009).} Electrons trapped in such dangling bond states at the surface of crystalline silicon have short retention times at room temperature, due to the proximity of the occupied state energy level to the conduction band. Here we report on a method for creating electron trap states at the surface of a silicon dioxide film by electron injection from a metalized Atomic Force Microscope probe tip. Single Electron Tunneling Force measurements\footnote{E. Bussmann, et. al. Appl. Phys. Lett., 85, 13 (2004).} are employed to examine the existence of trap states in the silicon dioxide surface before and after the electron injection. Evidence for electron trap state creation, without topographic modification of the silicon dioxide surface, will be presented. The trap states created by this process have electron retention times which are greater than one second at room temperature. The methodology for trap state creation and detection will be presented.   

Nm-scale Mapping of Thermally-Activated Trap Emission in an AlGaN/GaN High Electron Mobility Transistor

AlGaN/GaN high electron mobility transistors (HEMTs) are intrinsically ideal for high frequency and high power applications, but have degraded performance due to charge trapping. Nm-scale AFM-based electrical measurements sensitive to the emission of trapped charge, such as scanning Kelvin probe microscopy (SKPM), can, in principle, be used to determine the energies, cross sections, and densities of electrically-active traps with high spatial resolution. Using SKPM, we obtain nm-scale surface potential transient (SPT) maps over the entire surface of an AlGaN/GaN HEMT. Surprisingly, we find significant SPTs near the edges of the device that are similar to conductance transients, and whose time constants vary with temperature, indicating thermally-activated emission. Comparison of nm-scale measurements and electrostatic simulations will be discussed to quantify the spatial distribution of trapped charge near the edge of the device. Work supported by ONR-DRIFT (P. Maki).   

Stochastic switching of microcantilever motion

Fluctuation-induced transitions between two stable states of a strongly driven microcantilever are studied. Intrinsic geometric and inertial nonlinearities of the cantilever give rise to an amplitude-dependent resonance frequency, and at a critical point bifurcation occurs. Two states are stable, represented by vibration at a low and a high amplitude. Adding noise facilitates transitions between the states. The transition rate rises upon increasing noise intensity, as expected for noise-activated escape from a dynamic double well. Further increasing the noise intensity causes a parametric change in the dynamic double well, and results in a decay of the switching rate. Close to the onset of spontaneous transitions, the bistable cantilever is very sensitive. We demonstrate the noise-enhanced detection of weak modulations of the bistable cantilever, resulting in an optimum signal-to-noise ratio at non-minimum noise intensity.