Tip Apex Research Articles

Mechanism of nanostructure processing on Au and Ag nano-film by a nanosecond laser illuminating cantilevered scanning near-field optical microscopy tip.

Nanostructure processing by a laser illuminating cantilevered scanning near-field optical microscopy (SNOM) tip is a novel technology that has received extensive attention from researchers. In this paper, theoretical investigations of the mechanism for nanostructure fabrication on Au and Ag nano-film by this technology are realized by the finite element method. The light field intensity and temperature distribution on Au and Ag surfaces at the near-field of the SNOM tip apex after illumination is simulated. The results reveal that the laser is restricted and enhanced within the SNOM tip aperture during illumination. Locally excited surface plasmon polaritons, which induce near-field enhancement on the Au and Ag nano-film at the vicinity of the aperture, are significant for nanostructure fabrication. The impacts of several parameters such as aperture width w, gap between the apex and substrate g, and the initial electric field intensity |E0| of the laser on the temperature of the Au and Ag substrate surfaces during fabrication are deeply studied. It reveals that the surface temperature depends on both the enhancement of the light field intensity and the transmitted laser. The enhancement is dominant in affecting temperature when the gap is small, while the transmittance becomes the main factor influencing the surface temperature with the increase of the gap.

Nanoscale chiral imaging under complex optical field excitation with controllable oriented chiral dipole moment.

Since chirality is a fundamental building block of nature, the identification of the chiral specimen's structure is of great interest, especially in applications involving the modification and utilization of proteins. In this work, by exploiting photoinduced force exerted on an achiral tip placed in the vicinity of a reciprocal chiral sample, a novel technique is proposed to detect the sample's chirality in nanoscale spatial resolution. Under separate excitation of focal field carrying chiral dipole moment with opposite handedness, there is a differential optical force ΔF exerted on the tip apex, which is connected to the enantiomer type and quasi-linearly depends on specific component of the sample's chirality parameter. With the help of time-reversal approach, we prove that the required excitation can be derived by radiation fields from the superposition of parallel electric and magnetic dipoles. Through adjusting the orientation of the chiral dipole moment, all the diagonal components of the sample's chirality can be exclusively retrieved. In addition, the sensitivity of the proposed technique is demonstrated to enantiospecify nanoscale chiral samples with chirality parameter on the order of 0.001. The proposed technique may open new avenue for wide applications in biomedicine, material science and pharmaceutics.

Fabrication of high aspect ratio atomic force microscope probes using focused ion beam milled etch mask

The shape and dimension of atomic force microscope (AFM) probes are essential considerations to acquire high resolution images at the nanoscale. In the cases of nanostructures with high aspect ratio (HAR) features, including dot and line arrays, commercially available standard silicon AFM probes fail to provide satisfactory results. This is due to the low aspect ratio pyramidal tip profile which cannot adequately follow the sample surface. Here a simple method is introduced to convert commercially available pyramidal probes to HAR probes by combining focused ion beam (FIB) and plasma etching techniques. A bilayer metal is deposited on standard AFM probes using e-beam evaporation, followed by a quick FIB milling process to pattern the very thin top metal layer. The FIB milled metal layer is used as the hard mask to transfer the pattern to the underlying metal layer and an HAR pillar on top of the tip apex was achieved via plasma etching. Compared to the traditional HAR tip fabrication, where the tips are individually manufactured at high cost and in a time-consuming fashion, our method significantly reduces the usage of FIB milling. Additionally, this method is suitable for batch fabrication via plasma etching once the simple FIB milling step is performed.

Exploration of CO movement characteristics on rutile TiO2(110) surface

The adsorption and two-dimensional movement of CO molecules on the rutile TiO2(110) surface were investigated using atomic force microscopy at 78 K. We found that the stability of the CO molecule was affected around Oad. The CO molecule was unstable under short-range force and became stable, pinning it around Oad. The oxygen-terminated tip apex was used to surmount the force threshold of CO molecules to enable two-dimensional movement on the substrate, which can be achieved by short-range force. The CO adsorption and movement properties can be used for the catalytic design of CO oxide-based metal-semiconductor substrates.

Measurement of Tip Apex Distance and its Relation with Cut-Out After Fixation of Proximal Femoral Fractures with Dynamic Hip Screw or Proximal Femoral Nail A Retrospective Longitudinal Analysis of 106 Patients

Measurement of Tip Apex Distance and its Relation with Cut-Out After Fixation of Proximal Femoral Fractures with Dynamic Hip Screw or Proximal Femoral Nail A Retrospective Longitudinal Analysis of 106 Patients

Fast fabrication and judgement of tip-enhanced Raman spectroscopy-active tips

The quality of the scanning tip is crucial for tip-enhanced Raman spectroscopy (TERS) experiments towards large signal enhancement and high spatial resolution. In this work, we report a controllable fabrication method to prepare TERS-active tips by modifying the tip apex at the atomic scale, and propose two important criteria to in-situ judge the tip's TERS activity for tip-enhanced Raman measurements. One criterion is based on the downshift of the first image potential state to monitor the coupling between the far-field incident laser and near-field plasmon; the other is based on the appearance of the low-wavenumber Raman peaks associated with an atomistic protrusion at the tip apex to judge the coupling efficiency of emissions from the near field to the far field. This work provides an effective method to quickly fabricate and judge TERS-active tips before real TERS experiments on target molecules and other materials, which is believed to be instrumental for the development of TERS and other tip-enhanced spectroscopic techniques.

Design and Optimization of Tapered Optical Fiber Probes for SERS Utilizing FDTD Method

In this work, we report a strategy of Ag nanoparticle (Ag NP)-coated tapered optical fiber probes by finite difference time domain (FDTD) simulations. Investigation shows that the fiber-tip decorated Ag NPs has excellent electric field enhancement and confinement of light capabilities. Moreover, we demonstrate the effect of key parameters such as tip radius, conical angle, Ag NP size, and gaps between them on the field enhanced utilizing typical excitation wavelengths of 532, 633, and 785 nm. To further improve the electrical field effect, a noble metal substrate is introduced below the tip apex, which exhibits a higher field enhancement generated by tip-substrate coupling. The presence of the Au substrate does not lead to a significant change in the plasma characteristic peak of the probes at 490 nm. This study provides a useful reference for the fabrication of tapered optical fiber with plasmonic nanostructures and the design of robust tapered fiber-optic Raman sensors.

Nanoscale electric field imaging with an ambient scanning quantum sensor microscope

Nitrogen-vacancy (NV) center in diamond is a promising quantum sensor with remarkably versatile sensing capabilities. While scanning NV magnetometry is well-established, NV electrometry has been so far limited to bulk diamonds. Here we demonstrate imaging external alternating (AC) and direct (DC) electric fields with a single NV at the apex of a diamond scanning tip under ambient conditions. A strong electric field screening effect is observed at low frequencies. We quantitatively measure its frequency dependence and overcome this screening by mechanically oscillating the tip for imaging DC fields. Our scanning NV electrometry achieved an AC E-field sensitivity of 26 mV μm−1 Hz−1/2, a DC E-field gradient sensitivity of 2 V μm−2 Hz−1/2, and sub-100 nm resolution limited by the NV-sample distance. Our work represents an important step toward building a scanning-probe-based multimodal quantum sensing platform.

A straightforward optical alignment protocol for STM-based single molecule spectroscopy

A light-accessed scanning tunneling microscope (STM) is a powerful spectroscopic tool that enables chemical analysis at the single molecular level, but it requires highly precise optical alignments to pinpoint the nano-scale tunneling gap, leaving experimental challenges. Here we present straightforward procedures to align the optical setup for STM-luminescence and STM-based tip-enhanced Raman spectroscopy (TERS) performed with a reflection geometry in an ultrahigh vacuum chamber. Observing real-space images of the metal tip apex through a spectrograph set to the zeroth-order diffraction enables “in situ” optimization of the detection path and introduction of the excitation light of TERS to the nanogap. The best spatial overlap with the nanogap can be achieved by monitoring plasmon-enhanced, low-frequency inelastic scattering of the metal. This protocol allows us to overcome such difficulties in STM-based spectroscopy and facilitates physicochemical study of single adsorbates on nontransparent substrates.

Photovoltaic and photothermal effects induced by visible laser radiation in atomic force microscopy probes

Excitation of electrons into higher energy states in solid state materials can be induced by absorption of visible light, a physical process generally studied by optical absorption spectroscopy. A promising approach for improving the spatial resolution of optical absorption spectroscopy beyond the diffraction limit is the detection of photoinduced forces by an atomic force microscope operating under wavelength-dependent light irradiation. Here, we report on a combined photovoltaic/photothermal effect induced by the absorption of visible light by the microscope probes. By monitoring the photoinduced modifications of the oscillation of the probes, it is found that the oscillation phase-voltage parabolic signals display specific fingerprints which depend on light intensity and the nature of the materials composing the probes. In particular, a localized surface photovoltage (SPV) is evidenced at the tip apex of uncoated Si probes, while none is observed on Au-coated Si probes. The photothermal effects are distinguished from photovoltaic effects by specific shifts of the phase-voltage parabolas. The findings are relevant for the whole range of atomic force microscopy techniques making use of visible light as an additional means of local optical characterization.

Quantitative Magnetic Force Microscopy: Transfer-Function Method Revisited

Magnetic force microscopy (MFM) is a characterization method used to obtain images of the stray field emanating from the surface of magnetic samples with high spatial resolution. The MFM signal arises from a convolution of the magnetic moment distribution of the MFM tip with the stray field of the sample. Therefore, deconvolution of the stray field of a magnetic sample requires a well-defined micromagnetic structure of a tip. While the magnetic moment distribution of the tip remains challenging to assess, it is convenient to calibrate the tip-equivalent magnetic charge in a plane parallel to the sample surface running through the tip apex using a suitable tip-calibration method. Once the tip-equivalent magnetic charge is calibrated, MFM data can be deconvolved to obtain the stray field of the sample or the equivalent magnetic surface charge, for example, at the sample surface. Here we use low-noise MFM data obtained with high-quality-factor cantilevers and a modified L-curve method for the optimization of a Tikhonov deconvolution procedure used for the calibration of the MFM tip, and---once the tip is calibrated---for obtaining the stray field or magnetic surface charge of the sample.

Effective Stiffness of Hydrated Atomic Force Microscopy Tips.

When generating force curves with atomic force microscopy (AFM), the conventional assumption is that the silicon tip's apex is infinitely stiffer than the force gradient acting between the apex and test object. Although true for measurements in vacuum or at long distances, we show this assumption fails badly at short distances in aqueous environments. In this case, the effective apex is an adsorbed water molecule, bound by a weak O-H···O-H H-bond. At short distances, the magnitude of the force gradient exceeds the stiffness of this bond. This causes conventional AFM measurements to be dominated by the mechanical H-bond stiffness, instead of the force gradient. Here, we introduce a new multifrequency technique that is able to measure the surface force gradient independently from the H-bond. We compare our results to conventional FM-AFM and show that due to the H-bond, FM-AFM can give extremely erroneous measurements and even the wrong force polarity.

Improved durable vortex core MFM tip

• Vortex-Core probe offers long life time and stable magnetic moment. • Stability of vortex core in the disk increased for smaller diameter and larger disk thickness. • The stability of vortex core is improved by VC pinning at the disk center. • Asymmetry pinning center better pins VC than symmetrical one. • Probe with pinned VC gives 15 nm spatial resolution. Vortex-Core magnetic force microscopy (VC MFM) promises magnetic tip durability and long-term calibration of its magnetic moment that is crucial for quantitative scanning. It uses the vortex core of a ferromagnetic disk located at the tip apex as a magnetic probe. However, our experience showed that MFM images might be distorted when scanning magnetic objects larger than 100 nm due to a VC shift generated by the stray field of the scanned sample. We addressed the problem by changing the tip geometry and introducing artificial VC pinning at the disk center. These steps are essential for improving the VC tip for quantitative scanning. The scanning experiments with the optimized VC tip showed improved characteristics of the magnetic images – spatial resolution of the tip was on the level of commercial MFM tip with certain details depicted even better.

Numerical characterization of optical properties of tapered plasmonic structure on a cantilever pyramidal tip for plasmon nanofocusing

The plasmon nanofocusing process has been widely implemented in near-field scanning optical microscopy (NSOM) recently because it allows generating a background-free nanolight source at the apex of a metallic tip, enabling high contrast imaging at the nanoscale. In plasmon nanofocusing-assisted NSOM, the metallic tip properties play a vital role in generating an intense and well-confined nanolight source by controlling the plasmons’ behavior. This is why various tip designs have been developed so far. Recently, our group has also developed a metallic tapered tip, composed of a dielectric pyramidal base and a thin metallic layer coated on one side of the pyramid, using a novel fabrication method that allows tuning the optical properties of a tip depending on the requirement. Although our metallic tip has a unique advantage of tuning its optical properties, it has not yet been well studied. In this work, we present a thorough study of the optical properties of our metallic tip that depends on its parameters, such as the dielectric material, metal thickness, and cone angle, using finite-difference time-domain simulations. This particular study will allow us to understand controlling the tip’s optical properties and expand it for a wide range of applications.

Multi-scale modeling of an atomic force microscope tip for the study of frictional properties and oscillation behavior

In this paper, the vibrational behavior of the atomic force microscope (AFM) on a graphene sheet sample was analyzed using a multi-scale model. The cantilever and silicone tip base were simulated using continuum mechanics and finite element modeling, while the tip apex was modeled using Tersoff potential and structural mechanics modeling. The modified Morse potential was used to model the single-layer graphene, and the Lennard-jones potential was employed as nonlinear springs to model the interactions between the graphene layers and the tip-sample. In addition, the contact behavior between the tip and graphene was investigated by measuring the friction force during the movement of the tip on the graphene sheet, and the results were compared to those obtained from a molecular dynamics simulation and an experimental test. The friction force between the tip and graphene increased by enhancing the tip radius and the contact surface between the tip and the sample. With the initial distance displacement of the tip from the sample, two curves of the tip oscillation amplitude variations and the tip oscillation and excitation vibration phase shift were plotted. In conclusion, the results of the present multi-scale model are compared with those of the MD simulation and demonstrate the strong correlation between the proposed model and the MD model.

Comparison of Dynamic Versus Static Lag Screw Modes for Short Cephalomedullary Nails in the Treatment of Unstable Intertrochanteric Fractures: A Randomized Controlled Trial

Purpose: To compare the static locked and sliding proximal lag screw modes of short cephalomedullary nail in the treatment of unstable intertrochanteric fractures. Methods: Ninety-four patients (age>60 years) with low energy unstable intertrochanteric fractures were randomized for treatment into two groups. They were treated with static and sliding proximal lag screw modes of short cephalomedullary nail. The pre-operative variables, operative time, fluoroscopy time, blood loss, tip apex distance, and reduction quality were recorded for each patient. Post-operative follow-ups were undertaken every other week until bone union or implant failure occurred. Plain anteroposterior and lateral radiographs (both hip) were obtained at all visits. Ipsilateral leg length discrepancy (LLD), radiographic union score for hip complications, and fixation failure were recorded. Results: The mean follow-up time was 16.7 months (range 12–24). The mean bone union times of static locked (n=35) and sliding proximal (n=34) screw mode groups were 12.4 weeks and more than 11.2 weeks, respectively; the difference was not significant (p=0.213). The ipsilateral LLD of the sliding proximal (mean 4 mm) and static locked (mean 2 mm) screw mode groups showed a statistically significant difference (p<0.001). Post-operative complications (lag screw perforated to hip joint, lag screw cutout from the femoral head, and excessive inferolateral lag screw sliding) developed in 8.82% patients in sliding proximal group, whereas, no complications were reported in the static locked group. Conclusions: Treatment of unstable intertrochanteric fracture using static locked proximal lag screw mode of cephalomedullary nail showed some advantages over sliding proximal lag screw in terms of less complication and ipsilateral LLD; however, the bone union times were not different. Therefore, a static locked proximal screw mode is preferable over sliding proximal screw mode in treating unstable intertrochanteric fractures.

Is new always better: comparison of the femoral neck system and the dynamic hip screw in the treatment of femoral neck fractures

BackgroundHip fractures in the elderly population are common and the number of patients is rising. For young and geriatric patients with undisplaced fractures osteosynthesis is the primary type of treatment. The dynamic hip screw (DHS) is around for many years and proved its value especially in displaced fractures. Since 2018 the femoral neck system (FNS) is available as an alternative showing promising biomechanical results. The aim of this study is to evaluate clinical results of the FNS and compare it to the DHS.Materials and methodsPatients older than 18 years with Garden I–IV fractures that were treated with osteosynthesis in a level 1 trauma center were included in the study. Between January 2015 and March 2021, all patients treated with FNS (1-hole plate, DePuy-Synthes, Zuchwil, Switzerland) or DHS (2-hole plate, DePuy-Synthes, Zuchwil, Switzerland) for proximal femur fractures were included in the study. Closed reduction was achieved using a traction table. All operations were carried out by experienced orthopedic trauma surgeons. Primary outcome measures were rate of implant failure (cut out) and surgical complications (hematoma, infection). Secondary outcome measures were Hb-difference, length of hospital stay and mortality.ResultsOverall, 221 patients were included in the study. 113 were treated with FNS, 108 with DHS. Mean age was 69 ± 14 years. There were 17.2% Garden I, 47.5% Garden II, 26.7% Garden III and 8.6% Garden IV fractures. No difference between the groups for age, body mass index (BMI), Charlson comorbidity index (CCI), time to surgery, Pauwels and Garden classification, rate of optimal blade position or tip apex distance was found. FNS showed lower pre- to postoperative Hb-difference (1.4 ± 1.1 g/l vs. 2.1 ± 1.4 g/l; p < 0.05), shorter operating time (36.3 ± 11.6 min vs. 54.7 ± 17.4 min; p < 0.05) and hospital stay (8.8 ± 4.3 d vs. 11.2 ± 6.8 d; p < 0.05). Surgical complications (FNS 13.3% vs. DHS 18.4%, p > 0.05), rate of cut out (FNS 12.4% vs. DHS 10.2%, p > 0.05) and mortality (FNS 3.5%; DHS 0.9%; p > 0.05) showed no difference between the groups. Logistic regression showed that poor blade position was the only significant predictor for cut out and increased the risk by factor 7. Implant related infection (n = 3) and hematoma/seroma (n = 6) that needed revision was only seen in DHS group.ConclusionFNS proved to be as reliable as DHS in all patients with hip fractures. Not the type of implant but blade positioning is still key to prevent implant failure. Still due to minimal invasive approach implant related infections and postoperative hematomas might have been prevented using the FNS.

Sub-Nanometer Resolved Tip-Enhanced Raman Spectroscopy of a Single Molecule on the Si(111) Substrate

The investigation of molecular adsorption structures on nonmetallic surfaces is important for understanding the functionalities of nanodevices and biomolecules. In contrast to extensive tip-enhanced Raman spectroscopy (TERS) studies on metallic substrates with strong plasmonic enhancement, the studies on nonmetallic substrates like silicon (Si) are relatively limited. It remains to be clarified whether it is possible to detect the TERS signal from a single molecule directly adsorbed on a nonmetallic surface and how precise its spatial resolution could be achieved. Here we use Si(111) as a prototype nonmetallic substrate without plasmonic responses (in the visible region) and investigate the TERS of single bis(phthalocyaninato) terbium(III) (TbPc2) molecules adsorbed on this surface. We can not only detect the TERS signal from a single TbPc2 molecule, but also specify distinct adsorption configurations according to different TERS spectral features. The spatial resolution of TERS is also found down to ∼0.6 nm, which is slightly poorer than but still comparable with the resolution for the TbPc2 molecule directly adsorbed on the silver substrate. By combining with theoretical simulations, we find that although a plasmonic substrate can induce a larger TERS enhancement than a dielectric substrate, the subnanometer spatial resolution of TERS is mainly determined by the atomistic protrusion at the apex of a plasmonic metal tip, regardless of whether the substrate is plasmonic or dielectric. These findings offer a promise for applying subnanometer resolved TERS to a wide range of systems that contain nonmetallic substrates, such as in the semiconductor industry, 2D materials, and biomedical science.

Observation of robust superlubricity of MoS2 on Au(111) in ultrahigh vacuum

The structural and superlubric properties of single layer MoS2 on Au(111) forming moiré superlattice structures have been investigated by means of ultrahigh vacuum atomic force microscope with bimodal and contact modes. We synthesize epitaxial monolayer MoS2 flakes on the Au(111) surface in ultrahigh vacuum. Using friction force microscopy, atomic friction measurements indicate a superlubric regime between the tip apex and the moiré corrugated MoS2 surface in which the friction force remains at an ultralow value and is independent from normal load. Superlubricity conditions are observed for different loads and velocities which indicates the absence of out-of-plane deformations. We find that the MoS2 layer including the moiré superlattice modulation originating from the natural misfit between MoS2 and the Au(111) substrate is relatively rigid. We also demonstrate a low friction coefficient of the MoS2 surface crossing a single Au(111) step. Our results open up a new avenue for minimizing friction in nanoscale electronic devices and other dry rigid contacts used in aerospace lubrication.

Optimization of tip-enhanced Raman spectroscopy for probing the chemical structure of DNA

Tip-enhanced Raman (TER) spectroscopy combines the nanometric spatial resolution of atomic force microscopy (AFM) and the chemical sensitivity of Raman spectroscopy. Thus, it provides a unique possibility to obtain spectroscopic information on individual, nanometre-size molecules. The enhancement of Raman scattering cross-section requires modification of the AFM tip apex with a plasmonic nanostructure. Despite numerous advances of TERS research, attaining good reproducibility and stable enhancement is still challenging mainly due to the lack of optimized probes and sample preparation procedures. Moreover, current nanospectroscopic standard samples - carbon nanotubes (CNTs) have relatively simple chemical structure, and therefore, they are far from real-life analytes, especially biological samples. In this work we focus on the optimization of TERS technique for efficient DNA measurements, including: a preparation of atomically-flat gold substrates, fixative free deposition of DNA and optimization of TERS probe preparation. Here we demonstrate a comprehensive comparison of the efficacy of several types of TERS probes. Applying the systematic approach, we obtained reliable and reproducible TER spectra of DNA. Thus, we provide preparation procedures of a new standard TERS sample, TERS substrates and TERS probes. Our research provides a solid foundation for further research on DNA and its interaction with other biomolecules upon biologically significant processes such as DNA damage and repair.