Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid... more Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid scanning probe microscopy. By trapping air inside an encasement around the cantilever, they provide low damping and maintain high resonance frequencies for exquisitely low tip–sample interaction forces even when immersed in a viscous fluid. Quantitative measurements of stiffness, energy dissipation and tip–sample interactions using dynamic force sensors remain challenging due to spurious resonances of the system.Results: We demonstrate for the first time electrostatic actuation with a built-in electrode. Solely actuating the cantilever results in a frequency response free of spurious peaks. We analyze static, harmonic, and sub-harmonic actuation modes. Sub-harmonic mode results in stable amplitudes unaffected by potential offsets or fluctuations of the electrical surface potential. We present a simple plate capacitor model to describe the electrostatic actuation. The predicted deflectio...
Invading microbes face a myriad of cidal mechanisms of phagocytes that inflict physical damage to... more Invading microbes face a myriad of cidal mechanisms of phagocytes that inflict physical damage to microbial structures. How intracellular bacterial pathogens adapt to these stresses is not fully understood. Here, we report a new virulence mechanism by which mycobacteria alter the mechanical stiffness of their cell surface to become refractory to killing during infection. Long-Term Time-Lapse Atomic Force Microscopy was used to reveal a process of “mechanical morphotype switching” in mycobacteria exposed to host intracellular stress. A “soft” mechanical morphotype switch enhances tolerance to intracellular macrophage stress, including cathelicidin. Genetic manipulation, by deletion of uvrA, or pharmacological treatment, with bedaquiline, locked mycobacteria into a “soft” mechanical morphotype state, enhancing survival in macrophages. Our study proposes microbial mechanical adaptation as a new axis for surviving host-mediated stressors.One-Sentence SummaryBacteria alter their cell sur...
Scattering scanning near-field optical microscopy enables optical imaging and characterization of... more Scattering scanning near-field optical microscopy enables optical imaging and characterization of plasmonic devices with nanometer-scale resolution well below the diffraction limit. This technique enables developers to probe and understand the waveguide-coupled plasmonic antenna in as-fabricated heat-assisted magnetic recording heads. In order to validate and predict results and to extract information from experimental measurements that is physically comparable to simulations, a model was developed to translate the simulated electric field into expected near-field measurements using physical parameters specific to scattering scanning near-field optical microscopy physics. The methods used in this paper prove that scattering scanning near-field optical microscopy can be used to determine critical sub-diffraction-limited dimensions of optical field confinement, which is a crucial metrology requirement for the future of nano-optics, semiconductor photonic devices, and biological sensin...
22 We present Sensor Inpainting, a new operation mode for scanning probe microscopy that 23 uses ... more 22 We present Sensor Inpainting, a new operation mode for scanning probe microscopy that 23 uses advanced image processing techniques to render images based on position sensor 24 data. Sensor Inpainting frees scanning probe microscopy from the paradigm of raster 25 scanning, the scan waveforms do not need to fall on a grid, and the scanner is no longer 26 required to be at a specific location at a given time for each data point. This drastically 27 reduces the engineering effort of position control and enables the use of scan waveforms 28 that are better suited for the high inertia nanopositioners of scanned probe microscopy. 29 While in raster scanning, typically only trace or retrace images are used for display, in 30 Archimedean spiral scans 100% of the data can be displayed and at least a twofold 31 increase in temporal or spatial resolution is achieved. In Sensor Inpainting, the sampling 32 rate and grid size of the final generated image are independent variables. Sampling data...
We present a study of the effect of gold nanoparticles (Au NPs) on TiO 2 on charge generation and... more We present a study of the effect of gold nanoparticles (Au NPs) on TiO 2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. We used a novel characterization technique, photoassisted Kelvin probe force microscopy, to study the process at the single Au NP level. We found that the photoinduced electron transfer from TiO 2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron−hole pair generation in the space charge region in the TiO 2 −air interface and in the metal−semiconductor junction. Our measurements on single particles provide direct evidence for electron trapping that hinders electron−hole recombination, a key factor in the enhancement of photo(electro)catalytic activity.
Local conductance behavior of various moiré lattices, moiré superstructures, and Kagome-like moir... more Local conductance behavior of various moiré lattices, moiré superstructures, and Kagome-like moiré are resolved down to sub–5 nm.
Bilayer graphene was theorized to host a moiré miniband with flat dispersion if the layers are st... more Bilayer graphene was theorized to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the "magic angles" 1,2. Recently, such twisted bilayer graphene (tBLG) with the first magic angle twist was reported to exhibit correlated insulating state and superconductivity 3,4 , where the presence of the flat miniband in the system is thought to be essential for the emergence of these ordered phases in the transport measurements. Tunneling spectroscopy 5-9 and electronic compressibility measurements 10 in tBLG have revealed a van Hove singularity that is consistent with the presence of the flat miniband. However, a direct observation of the flat dispersion in the momentumspace of such moiré miniband in tBLG is still elusive. Here, we report the visualization of the flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution (nanoARPES). The high spatial resolution in nanoARPES enabled the measurement of the local electronic structure of the tBLG. We clearly demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.
We propose a new scan waveform ideally suited for high-speed Atomic Force Microscopy. It is an op... more We propose a new scan waveform ideally suited for high-speed Atomic Force Microscopy. It is an optimization of the Archimedean spiral scan path with respect to the X,Y scanner bandwidth and scan speed. The resulting waveform uses a constant angular velocity spiral in the center and transitions to constant linear velocity towards the periphery of the scan. We compare it with other scan paths and demonstrate that our novel spiral best satisfies the requirements of high-speed Atomic Force Microscopy by utilizing the scan time most efficiently with excellent data density and data distribution. For accurate X,Y, and Z positioning our proposed scan pattern has low angular frequency and low linear velocities that respect the instruments mechanical limits. Using Sensor Inpainting we show artifact free high resolution images taken at two frames per second with a 2.2 m scan size on a moderately large scanner capable of 40 m scans.
Nanoparticle-surfactants (NPSs) assembled at water−oil interfaces can significantly lower the int... more Nanoparticle-surfactants (NPSs) assembled at water−oil interfaces can significantly lower the interfacial tension and can be used to stabilize liquids. Knowing the formation and assembly and actively tuning the packing of these NPSs is of significant fundamental interest for the interfacial behavior of nanoparticles and of interest for water purification, drug encapsulation, enhanced oil recovery, and innovative energy transduction applications. Here, we demonstrate by means of interfacial tension measurements the high ionic strength helps the adsorption of NPSs to the water−oil interface leading to a denser packing of NPSs at the interface. With the reduction of interfacial area, the phase transitions from a "gas"like to "liquid" to "solid" states of NPSs in two dimensions are observed. Finally, we provide the first in situ real-space imaging of NPSs at the water−oil interface by atomic force microcopy.
Enamel, the outermost layer of teeth, is an acellular mineralized tissue that cannot regenerate; ... more Enamel, the outermost layer of teeth, is an acellular mineralized tissue that cannot regenerate; the mature tissue is composed of high aspect ratio apatite nanocrystals organized into rods and inter-rod regions. Amelogenin constitutes 90% of the protein matrix in developing enamel and plays a central role in guiding the hierarchical organization of apatite crystals observed in mature enamel. To date, a convincing link between amelogenin supramolecular structures and mature enamel has yet to be described, in part because the protein matrix is degraded during tissue maturation. Here we show compelling evidence that amelogenin self-assembles into an amyloid-like structure in vitro and in vivo. We show that enamel matrices stain positive for amyloids and we identify a specific region within amelogenin that self-assembles into β-sheets. We propose that amelogenin nanoribbons template the growth of apatite mineral in human enamel. This is a paradigm shift from the current model of enamel ...
Carbon nanotubes are potentially ideal atomic force microscopy (AFM) probes due to their well-def... more Carbon nanotubes are potentially ideal atomic force microscopy (AFM) probes due to their well-defined geometry, robust mechanical properties, and in the case of single-walled nanotubes (SWNTs), diameters approaching the size of small organic molecules. Here we elucidate fundamental factors that determine AFM imaging resolution using a combination of experiments with well-characterized SWNT probes and numerical simulations. Tapping mode AFM imaging on gold nanoclusters and DNA molecules using SWNT probes shows that the tip-induced broadening reaches the geometrical limit (i.e., the diameter of the SWNT tips) in the repulsive imaging regime, and the broadening effect becomes greater than the geometrical limit in the attractive imaging regime due to long-range attractive forces. Adjusting the damping enables imaging of isolated DNA molecules with a resolution defined by the tip diameter without disrupting the sample.
A critical limitation of Nano-electromechanical systems (NEMS) is the lack of a high sensitivity ... more A critical limitation of Nano-electromechanical systems (NEMS) is the lack of a high sensitivity position detection mechanism. We introduce a non-interferometric optical approach to determine the position of nanowires with high sensitivity and bandwidth. Its physical origins and limitations are determined by Mie scattering analysis. This enables a dramatic miniaturization of detectable cantilevers, with attendant reductions to the fundamental minimum force noise in highly damping environments. We measure the force-noise of an 80nm radius Ag2Ga nanowire-cantilever in water at 1.9 f N/ √ Hz.
Sub-Ångstrom-resolution sensing of tiny cantilevers is leading the way to a gentle physical probe... more Sub-Ångstrom-resolution sensing of tiny cantilevers is leading the way to a gentle physical probe for surfaces of materials such as living cells.
Investigation into the origin of forces dates to the early Greeks. Yet, only in recent decades ha... more Investigation into the origin of forces dates to the early Greeks. Yet, only in recent decades have techniques for elucidating the molecular origin of forces been developed. Specifically, Chemical Force Microscopy uses the high precision and nanometer scale probe of Atomic Force Microscopy to measure molecular and interfacial interactions. This thesis presents the development of many novel Chemical Force Microscopy techniques for measuring equilibrium and time-dependant force profiles of molecular interactions, which led to a greater understanding of the origin of interfacial forces in solution. In chapter 2, Magnetic Feedback Chemical Force Microscopy stiffens the cantilever for measuring force profiles between self-assembled monolayer (SAM) surfaces. Hydroxyl and carboxyl terminated SAMs produce long-range interactions that extend one or three nanometers into the solvent, respectively. In chapter 3, an ultra low noise AFM is produced through multiple modifications to the optical d...
The reaction of ozone with the (100) plane of solid potassium iodide (KI) was investigated using ... more The reaction of ozone with the (100) plane of solid potassium iodide (KI) was investigated using atomic force microscopy (AFM). The reaction forming potassium iodate (KIO 3) initiates at step edges prior to reacting on the flat terraces. Small domains of KIO 3 , initially 3.8 Å in height are formed on the top of step edges. Following reaction at the step edge, domains of KIO 3 are formed across the terraces. With prolonged exposure to ozone, KIO 3 domains nucleate further growth until the surface is evenly covered with KIO 3 particles that are 4-6 nm in height, at which point the surface is passivated and the reaction terminates. Introduction. Gas-phase halogens and halogenated oxides (X, XO, X 2 , XY, OXO, HOX, XONO 2 , XNO 2 , where X,Y=Cl, Br, I) affect the composition of the atmosphere as they play prominent roles in key events such as the oxidative capacity of the atmosphere, the global ozone budget, and aerosol nucleation and growth. 1,2 The fundamental mechanisms responsible for gas-phase halogen release in the atmosphere are not well understood. Sea-salt aerosols have been shown to readily react with oxidants such as ozone and the subsequent reactions are suggested as a possible mechanism for the introduction of gas-phase reactive molecular halogens into the troposphere. 3 Sea-salt droplets are present in the troposphere as aerosols, generated by wave-action in environments where the relative humidity exceeds that of their deliquescence point. In inland environments sea-salt particles are also found in the troposphere where they remain covered in thin water films. In addition, sea-salt ice is important in the polar-regions. 4 Experimental studies have shown that in bromide-doped NaCl single crystals, at doping ratios as low as 0.001 mol%, Bris greatly enhanced at the surface following subsequent drying of the deliquesced particle. 5, 6 As such, sea-salt aerosols that are transported to inland environments of low relative humidity are expected to retain the interfacial enhancement of the larger more polarizable anions that have been predicted theoretically and confirmed experimentally in aqueous solutions. 7-9 The surfaces of solid crystalline sea-salt particles are similarly expected to have a much higher concentration of iodide than in the bulk. Previous x-ray photoelectron spectroscopy (XPS) studies have shown, under conditions in which a low defect density KI surface is completely devoid of adsorbed water molecules, that surface oxidation by ozone results in a self-passivating layer of KIO 3. 10 XPS provides chemical identification of the reacting surface and products but is unable to provide spatial characterization of the reacting interface. Fluidized-bed reactor studies have shown the uptake of ozone to scale with iodide surface area, consistent with surface passivation. 11 AFM is a non-destructive local probe technique that provides topographical and chemical information about the surface with atomic resolution. AFM imaging complements the previous XPS studies and provides a detailed microscopic understanding of the reacting interface. Herein, we present two fundamental results from the model system, KI (s) + 3O 3(g) → KIO 3(s) + 3O 2(g). First, the oxidation of dry KI to form KIO 3 initiates at step edges before reacting across sample terraces. Second, a passivating layer is formed when small particles of KIO 3 , typically 10-30 nm in width and 4-6 nm in height are tightly packed across the sample surface creating a surface with a RMS roughness of 3.2 nm. Experimental. The experimental details of the XPS studies (carried out at beamline 9.3.2 of the Advanced Light Source) have been described elsewhere. 10,12 Solid samples of KI (Hilger
We remark that future advances in the synthesis of tetrapods will lead to better shape control, a... more We remark that future advances in the synthesis of tetrapods will lead to better shape control, and in particular to tetrapods with longer arms. This will increase the performance of tetrapod-functionalized tips in terms of aspect ratio. In another direction, AFM probes that are functionalized with semiconductor nanocrystals can lead to applications that sense not only the topography, but also the optical or electronic properties of a sample. In conclusion, we have demonstrated a quick and simple method to position single tetrapods at the extremity of flattened AFM tips. Such tetrapod-functionalized tips show very good AFM imaging performance with respect to spatial resolution and aspect ratio.
We demonstrate two patterning techniques based on hydrogen passivation of Ga 1-x Mn x As to produ... more We demonstrate two patterning techniques based on hydrogen passivation of Ga 1-x Mn x As to produce isolated ferromagnetically active regions embedded uniformly in a paramagnetic, insulating host. The first method consists of selective hydrogenation of Ga 1-x Mn x As by lithographic masking. Magnetotransport measurements of Hall-bars made in this manner display the characteristic properties of the hole-mediated ferromagnetic phase, which result from good pattern isolation. Arrays of Ga 1-x Mn x As dots as small as 250 nm across have been realized by this process. The second process consists of blanket hydrogenation of Ga 1-x Mn x As followed by local reactivation using confined low-power pulsed-laser annealing. Conductance imaging reveals local electrical reactivation of micrometer-sized regions that accompanies the restoration of ferromagnetism. The spatial resolution achievable with this method can potentially reach <100 nm by employing near-field laser processing. The high spatial resolution attainable by hydrogenation patterning enables the development of systems with novel functionalities such as lateral spin-injection as well as the exploration of magnetization dynamics in individual and coupled structures made from this novel class of semiconductors.
Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid... more Background: Encased cantilevers are novel force sensors that overcome major limitations of liquid scanning probe microscopy. By trapping air inside an encasement around the cantilever, they provide low damping and maintain high resonance frequencies for exquisitely low tip–sample interaction forces even when immersed in a viscous fluid. Quantitative measurements of stiffness, energy dissipation and tip–sample interactions using dynamic force sensors remain challenging due to spurious resonances of the system.Results: We demonstrate for the first time electrostatic actuation with a built-in electrode. Solely actuating the cantilever results in a frequency response free of spurious peaks. We analyze static, harmonic, and sub-harmonic actuation modes. Sub-harmonic mode results in stable amplitudes unaffected by potential offsets or fluctuations of the electrical surface potential. We present a simple plate capacitor model to describe the electrostatic actuation. The predicted deflectio...
Invading microbes face a myriad of cidal mechanisms of phagocytes that inflict physical damage to... more Invading microbes face a myriad of cidal mechanisms of phagocytes that inflict physical damage to microbial structures. How intracellular bacterial pathogens adapt to these stresses is not fully understood. Here, we report a new virulence mechanism by which mycobacteria alter the mechanical stiffness of their cell surface to become refractory to killing during infection. Long-Term Time-Lapse Atomic Force Microscopy was used to reveal a process of “mechanical morphotype switching” in mycobacteria exposed to host intracellular stress. A “soft” mechanical morphotype switch enhances tolerance to intracellular macrophage stress, including cathelicidin. Genetic manipulation, by deletion of uvrA, or pharmacological treatment, with bedaquiline, locked mycobacteria into a “soft” mechanical morphotype state, enhancing survival in macrophages. Our study proposes microbial mechanical adaptation as a new axis for surviving host-mediated stressors.One-Sentence SummaryBacteria alter their cell sur...
Scattering scanning near-field optical microscopy enables optical imaging and characterization of... more Scattering scanning near-field optical microscopy enables optical imaging and characterization of plasmonic devices with nanometer-scale resolution well below the diffraction limit. This technique enables developers to probe and understand the waveguide-coupled plasmonic antenna in as-fabricated heat-assisted magnetic recording heads. In order to validate and predict results and to extract information from experimental measurements that is physically comparable to simulations, a model was developed to translate the simulated electric field into expected near-field measurements using physical parameters specific to scattering scanning near-field optical microscopy physics. The methods used in this paper prove that scattering scanning near-field optical microscopy can be used to determine critical sub-diffraction-limited dimensions of optical field confinement, which is a crucial metrology requirement for the future of nano-optics, semiconductor photonic devices, and biological sensin...
22 We present Sensor Inpainting, a new operation mode for scanning probe microscopy that 23 uses ... more 22 We present Sensor Inpainting, a new operation mode for scanning probe microscopy that 23 uses advanced image processing techniques to render images based on position sensor 24 data. Sensor Inpainting frees scanning probe microscopy from the paradigm of raster 25 scanning, the scan waveforms do not need to fall on a grid, and the scanner is no longer 26 required to be at a specific location at a given time for each data point. This drastically 27 reduces the engineering effort of position control and enables the use of scan waveforms 28 that are better suited for the high inertia nanopositioners of scanned probe microscopy. 29 While in raster scanning, typically only trace or retrace images are used for display, in 30 Archimedean spiral scans 100% of the data can be displayed and at least a twofold 31 increase in temporal or spatial resolution is achieved. In Sensor Inpainting, the sampling 32 rate and grid size of the final generated image are independent variables. Sampling data...
We present a study of the effect of gold nanoparticles (Au NPs) on TiO 2 on charge generation and... more We present a study of the effect of gold nanoparticles (Au NPs) on TiO 2 on charge generation and trapping during illumination with photons of energy larger than the substrate band gap. We used a novel characterization technique, photoassisted Kelvin probe force microscopy, to study the process at the single Au NP level. We found that the photoinduced electron transfer from TiO 2 to the Au NP increases logarithmically with light intensity due to the combined contribution of electron−hole pair generation in the space charge region in the TiO 2 −air interface and in the metal−semiconductor junction. Our measurements on single particles provide direct evidence for electron trapping that hinders electron−hole recombination, a key factor in the enhancement of photo(electro)catalytic activity.
Local conductance behavior of various moiré lattices, moiré superstructures, and Kagome-like moir... more Local conductance behavior of various moiré lattices, moiré superstructures, and Kagome-like moiré are resolved down to sub–5 nm.
Bilayer graphene was theorized to host a moiré miniband with flat dispersion if the layers are st... more Bilayer graphene was theorized to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the "magic angles" 1,2. Recently, such twisted bilayer graphene (tBLG) with the first magic angle twist was reported to exhibit correlated insulating state and superconductivity 3,4 , where the presence of the flat miniband in the system is thought to be essential for the emergence of these ordered phases in the transport measurements. Tunneling spectroscopy 5-9 and electronic compressibility measurements 10 in tBLG have revealed a van Hove singularity that is consistent with the presence of the flat miniband. However, a direct observation of the flat dispersion in the momentumspace of such moiré miniband in tBLG is still elusive. Here, we report the visualization of the flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution (nanoARPES). The high spatial resolution in nanoARPES enabled the measurement of the local electronic structure of the tBLG. We clearly demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.
We propose a new scan waveform ideally suited for high-speed Atomic Force Microscopy. It is an op... more We propose a new scan waveform ideally suited for high-speed Atomic Force Microscopy. It is an optimization of the Archimedean spiral scan path with respect to the X,Y scanner bandwidth and scan speed. The resulting waveform uses a constant angular velocity spiral in the center and transitions to constant linear velocity towards the periphery of the scan. We compare it with other scan paths and demonstrate that our novel spiral best satisfies the requirements of high-speed Atomic Force Microscopy by utilizing the scan time most efficiently with excellent data density and data distribution. For accurate X,Y, and Z positioning our proposed scan pattern has low angular frequency and low linear velocities that respect the instruments mechanical limits. Using Sensor Inpainting we show artifact free high resolution images taken at two frames per second with a 2.2 m scan size on a moderately large scanner capable of 40 m scans.
Nanoparticle-surfactants (NPSs) assembled at water−oil interfaces can significantly lower the int... more Nanoparticle-surfactants (NPSs) assembled at water−oil interfaces can significantly lower the interfacial tension and can be used to stabilize liquids. Knowing the formation and assembly and actively tuning the packing of these NPSs is of significant fundamental interest for the interfacial behavior of nanoparticles and of interest for water purification, drug encapsulation, enhanced oil recovery, and innovative energy transduction applications. Here, we demonstrate by means of interfacial tension measurements the high ionic strength helps the adsorption of NPSs to the water−oil interface leading to a denser packing of NPSs at the interface. With the reduction of interfacial area, the phase transitions from a "gas"like to "liquid" to "solid" states of NPSs in two dimensions are observed. Finally, we provide the first in situ real-space imaging of NPSs at the water−oil interface by atomic force microcopy.
Enamel, the outermost layer of teeth, is an acellular mineralized tissue that cannot regenerate; ... more Enamel, the outermost layer of teeth, is an acellular mineralized tissue that cannot regenerate; the mature tissue is composed of high aspect ratio apatite nanocrystals organized into rods and inter-rod regions. Amelogenin constitutes 90% of the protein matrix in developing enamel and plays a central role in guiding the hierarchical organization of apatite crystals observed in mature enamel. To date, a convincing link between amelogenin supramolecular structures and mature enamel has yet to be described, in part because the protein matrix is degraded during tissue maturation. Here we show compelling evidence that amelogenin self-assembles into an amyloid-like structure in vitro and in vivo. We show that enamel matrices stain positive for amyloids and we identify a specific region within amelogenin that self-assembles into β-sheets. We propose that amelogenin nanoribbons template the growth of apatite mineral in human enamel. This is a paradigm shift from the current model of enamel ...
Carbon nanotubes are potentially ideal atomic force microscopy (AFM) probes due to their well-def... more Carbon nanotubes are potentially ideal atomic force microscopy (AFM) probes due to their well-defined geometry, robust mechanical properties, and in the case of single-walled nanotubes (SWNTs), diameters approaching the size of small organic molecules. Here we elucidate fundamental factors that determine AFM imaging resolution using a combination of experiments with well-characterized SWNT probes and numerical simulations. Tapping mode AFM imaging on gold nanoclusters and DNA molecules using SWNT probes shows that the tip-induced broadening reaches the geometrical limit (i.e., the diameter of the SWNT tips) in the repulsive imaging regime, and the broadening effect becomes greater than the geometrical limit in the attractive imaging regime due to long-range attractive forces. Adjusting the damping enables imaging of isolated DNA molecules with a resolution defined by the tip diameter without disrupting the sample.
A critical limitation of Nano-electromechanical systems (NEMS) is the lack of a high sensitivity ... more A critical limitation of Nano-electromechanical systems (NEMS) is the lack of a high sensitivity position detection mechanism. We introduce a non-interferometric optical approach to determine the position of nanowires with high sensitivity and bandwidth. Its physical origins and limitations are determined by Mie scattering analysis. This enables a dramatic miniaturization of detectable cantilevers, with attendant reductions to the fundamental minimum force noise in highly damping environments. We measure the force-noise of an 80nm radius Ag2Ga nanowire-cantilever in water at 1.9 f N/ √ Hz.
Sub-Ångstrom-resolution sensing of tiny cantilevers is leading the way to a gentle physical probe... more Sub-Ångstrom-resolution sensing of tiny cantilevers is leading the way to a gentle physical probe for surfaces of materials such as living cells.
Investigation into the origin of forces dates to the early Greeks. Yet, only in recent decades ha... more Investigation into the origin of forces dates to the early Greeks. Yet, only in recent decades have techniques for elucidating the molecular origin of forces been developed. Specifically, Chemical Force Microscopy uses the high precision and nanometer scale probe of Atomic Force Microscopy to measure molecular and interfacial interactions. This thesis presents the development of many novel Chemical Force Microscopy techniques for measuring equilibrium and time-dependant force profiles of molecular interactions, which led to a greater understanding of the origin of interfacial forces in solution. In chapter 2, Magnetic Feedback Chemical Force Microscopy stiffens the cantilever for measuring force profiles between self-assembled monolayer (SAM) surfaces. Hydroxyl and carboxyl terminated SAMs produce long-range interactions that extend one or three nanometers into the solvent, respectively. In chapter 3, an ultra low noise AFM is produced through multiple modifications to the optical d...
The reaction of ozone with the (100) plane of solid potassium iodide (KI) was investigated using ... more The reaction of ozone with the (100) plane of solid potassium iodide (KI) was investigated using atomic force microscopy (AFM). The reaction forming potassium iodate (KIO 3) initiates at step edges prior to reacting on the flat terraces. Small domains of KIO 3 , initially 3.8 Å in height are formed on the top of step edges. Following reaction at the step edge, domains of KIO 3 are formed across the terraces. With prolonged exposure to ozone, KIO 3 domains nucleate further growth until the surface is evenly covered with KIO 3 particles that are 4-6 nm in height, at which point the surface is passivated and the reaction terminates. Introduction. Gas-phase halogens and halogenated oxides (X, XO, X 2 , XY, OXO, HOX, XONO 2 , XNO 2 , where X,Y=Cl, Br, I) affect the composition of the atmosphere as they play prominent roles in key events such as the oxidative capacity of the atmosphere, the global ozone budget, and aerosol nucleation and growth. 1,2 The fundamental mechanisms responsible for gas-phase halogen release in the atmosphere are not well understood. Sea-salt aerosols have been shown to readily react with oxidants such as ozone and the subsequent reactions are suggested as a possible mechanism for the introduction of gas-phase reactive molecular halogens into the troposphere. 3 Sea-salt droplets are present in the troposphere as aerosols, generated by wave-action in environments where the relative humidity exceeds that of their deliquescence point. In inland environments sea-salt particles are also found in the troposphere where they remain covered in thin water films. In addition, sea-salt ice is important in the polar-regions. 4 Experimental studies have shown that in bromide-doped NaCl single crystals, at doping ratios as low as 0.001 mol%, Bris greatly enhanced at the surface following subsequent drying of the deliquesced particle. 5, 6 As such, sea-salt aerosols that are transported to inland environments of low relative humidity are expected to retain the interfacial enhancement of the larger more polarizable anions that have been predicted theoretically and confirmed experimentally in aqueous solutions. 7-9 The surfaces of solid crystalline sea-salt particles are similarly expected to have a much higher concentration of iodide than in the bulk. Previous x-ray photoelectron spectroscopy (XPS) studies have shown, under conditions in which a low defect density KI surface is completely devoid of adsorbed water molecules, that surface oxidation by ozone results in a self-passivating layer of KIO 3. 10 XPS provides chemical identification of the reacting surface and products but is unable to provide spatial characterization of the reacting interface. Fluidized-bed reactor studies have shown the uptake of ozone to scale with iodide surface area, consistent with surface passivation. 11 AFM is a non-destructive local probe technique that provides topographical and chemical information about the surface with atomic resolution. AFM imaging complements the previous XPS studies and provides a detailed microscopic understanding of the reacting interface. Herein, we present two fundamental results from the model system, KI (s) + 3O 3(g) → KIO 3(s) + 3O 2(g). First, the oxidation of dry KI to form KIO 3 initiates at step edges before reacting across sample terraces. Second, a passivating layer is formed when small particles of KIO 3 , typically 10-30 nm in width and 4-6 nm in height are tightly packed across the sample surface creating a surface with a RMS roughness of 3.2 nm. Experimental. The experimental details of the XPS studies (carried out at beamline 9.3.2 of the Advanced Light Source) have been described elsewhere. 10,12 Solid samples of KI (Hilger
We remark that future advances in the synthesis of tetrapods will lead to better shape control, a... more We remark that future advances in the synthesis of tetrapods will lead to better shape control, and in particular to tetrapods with longer arms. This will increase the performance of tetrapod-functionalized tips in terms of aspect ratio. In another direction, AFM probes that are functionalized with semiconductor nanocrystals can lead to applications that sense not only the topography, but also the optical or electronic properties of a sample. In conclusion, we have demonstrated a quick and simple method to position single tetrapods at the extremity of flattened AFM tips. Such tetrapod-functionalized tips show very good AFM imaging performance with respect to spatial resolution and aspect ratio.
We demonstrate two patterning techniques based on hydrogen passivation of Ga 1-x Mn x As to produ... more We demonstrate two patterning techniques based on hydrogen passivation of Ga 1-x Mn x As to produce isolated ferromagnetically active regions embedded uniformly in a paramagnetic, insulating host. The first method consists of selective hydrogenation of Ga 1-x Mn x As by lithographic masking. Magnetotransport measurements of Hall-bars made in this manner display the characteristic properties of the hole-mediated ferromagnetic phase, which result from good pattern isolation. Arrays of Ga 1-x Mn x As dots as small as 250 nm across have been realized by this process. The second process consists of blanket hydrogenation of Ga 1-x Mn x As followed by local reactivation using confined low-power pulsed-laser annealing. Conductance imaging reveals local electrical reactivation of micrometer-sized regions that accompanies the restoration of ferromagnetism. The spatial resolution achievable with this method can potentially reach <100 nm by employing near-field laser processing. The high spatial resolution attainable by hydrogenation patterning enables the development of systems with novel functionalities such as lateral spin-injection as well as the exploration of magnetization dynamics in individual and coupled structures made from this novel class of semiconductors.
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Papers by Paul Ashby