Tip Apex Research Articles

Numerical Characterization of Magnetic Vortex Probe Imaging for Magnetic Force Microscopy

We study theoretically the performance and limitations of the magnetic vortex probe for magnetic force microscopy (MFM). In the ideal case, the only magnetically active part of the probe is the magnetic vortex core (VC) existing in the center of a Permalloy (Py) disk located at the apex of a non-magnetic tip. Such VC can be effectively characterized as a point dipole with the magnetic moment of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-17</sup> Am <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> embedded about 20 nm inside the disk, which is similar to the commercial low-momentum MFM probes. In addition to the standard probes, the ideal VC probe offers high durability and a well-controlled magnetic moment suitable for quantitative MFM. However, since the VC probe is made of magnetically soft material its magnetization profile and the resulting MFM image can be deformed by the stray field of the sample. Therefore, the VC probe is suited for imaging the samples with small domain sizes which generate low stray fields. We numerically examine VC imaging of typical magnetic samples, i.e. domain arranged as a single circular dot, stripe patterns, and the chessboard. We determine the limiting dimensions of the domains that can be correctly imaged by the VC tip of optimal parameters. In general, we can conclude that MFM imaging by VC probes gives reasonable results for the samples with domains with lateral dimensions up to 100 nm.

Is Nail-Canal Diameter Discordance a Risk Factor for the Excessive Sliding of Cephalomedullary Nails in Geriatric Intertrochanteric Fracture Surgery?

Background and Objectives:: There were limited studies which investigated nail diameter as a predictor for cephalomedullary nail (CMN) failure in intertrochanteric fracture (ITF). We aimed to evaluate the surgical outcomes of CMN in fragility ITF following nail-canal (N-C) diameter discordance. Materials and Methods: From November 2010 to March 2022, we retrospectively reviewed 120 consecutive patients who underwent CMN surgeries due to fragility ITF. We included patients with acceptable reduction and a tip-apex distance ≤ 25 mm. The N-C diameter differences both in anterior-posterior (AP) and lateral-view X-rays were measured, and we compared the number of excessive sliding instances and the rate of implant failure between the N-C concordance (≤3 mm) and discordance (>3 mm) group. Simple linear regression was used to determine the strength of the relationship between the N-C difference and sliding distance. Results: The sliding distance showed no differences between the groups in the AP (3.6 vs. 3.3 mm, p = 0.75) and lateral view (3.5 vs. 3.4 mm, p = 0.91). For analyses in the AP view, the AP-concordance and AP-discordance groups had 14 (25%) and 14 patients (22%) with a sliding distance of >5 mm (p = 0.69), while treatment failure occurred in 3 (5%) and 3 (3%) patients, respectively (p = 0.66). For analyses in the lateral view, the lat-concordance and lat-discordance groups had 8 (27%) and 20 patients (22%) with a sliding distance of >5 mm (p = 0.62), while treatment failure occurred in 1 (3%) and 4 (4%) patients, respectively (p = 1.00). Linear regression analyses showed that the N-C difference in either views was not a significant predictor of sliding distance in both the AP (R2 = 0.002, p = 0.60) and lateral views (R2 = 0.007, p = 0.35). Conclusions: If appropriate fracture reduction and fixation are achieved, the N-C discordance of short CMN does not affect treatment outcomes in ITF.

Molecular Identification from AFM Images Using the IUPAC Nomenclature and Attribute Multimodal Recurrent Neural Networks.

Spectroscopic methods─like nuclear magnetic resonance, mass spectrometry, X-ray diffraction, and UV/visible spectroscopies─applied to molecular ensembles have so far been the workhorse for molecular identification. Here, we propose a radically different chemical characterization approach, based on the ability of noncontact atomic force microscopy with metal tips functionalized with a CO molecule at the tip apex (referred as HR-AFM) to resolve the internal structure of individual molecules. Our work demonstrates that a stack of constant-height HR-AFM images carries enough chemical information for a complete identification (structure and composition) of quasiplanar organic molecules, and that this information can be retrieved using machine learning techniques that are able to disentangle the contribution of chemical composition, bond topology, and internal torsion of the molecule to the HR-AFM contrast. In particular, we exploit multimodal recurrent neural networks (M-RNN) that combine convolutional neural networks for image analysis and recurrent neural networks to deal with language processing, to formulate the molecular identification as an imaging captioning problem. The algorithm is trained using a data set─which contains almost 700,000 molecules and 165 million theoretical AFM images─to produce as final output the IUPAC name of the imaged molecule. Our extensive test with theoretical images and a few experimental ones shows the potential of deep learning algorithms in the automatic identification of molecular compounds by AFM. This achievement supports the development of on-surface synthesis and overcomes some limitations of spectroscopic methods in traditional solution-based synthesis.

To study correlation between the tip apex distance and the functional outcome of fracture intertrochanteric femur treated with trochanteric femoral nail

Introduction: Tip Apex Distance (TAD) is well established method of prediction of outcome in intertrochanteric fractures managed by Dynamic Hip Screw. This retrospective study was done to assess the significance of Tip Apex Distance in the management of the intertrochanteric fractures by proximal femoral nail.Methodology: Selection of Cases-An informed written consent will be taken from all the patient/relatives/guardian after the approval of Institutional Ethics Committee.Source of Data: Patients admitted to Sri Aurobindo Medical College and PG Institute, Indore for fracture proximal humerus in adult patients.Conclusion: This study indicates that the Tip Apex Distance can be used as a useful predictor of the outcome of the proximal femoral nail in intertrochanteric fractures.

Intramedullary Femur Nailing in Intertrochanteric Fractures: Postoperatively Do Helical Blades Migrate More Than Lag Screws? A Randomized Controlled Trial.

This was a prospective, randomized controlled trial to test functional outcomes between a single lag screw and helical blade nails in the treatment of intertrochanteric fractures. 72 patients with intertrochanteric fractures between March 2019 to November 2020 were randomized into two groups and treated with a lag screw or a helical blade nail. Intraoperative parameters such as operative time, blood loss, and radiation exposure were calculated. Postoperatively, tip apex distance, neck length, neck-shaft angle, lateral impingement of implant, union rate, and the functional outcomes were measured at the end of 6month follow-up period. There was a significant decrease in tip apex distance (p = 0.03) and neck length(p-0.04) with significant lateral impingement of the implant (p = 0.04) in the helical blade group compared to the lag screw group. The functional outcome calculated using the modified Harris Hip score & Parker and Palmer mobility score, at the end of 6months, had no significant difference between the two groups. Both lag screw and helical blade devices can be used to successfully treat these fractures, although there is greater medial migration in the helical blade as compared to the lag screw.

A biomechanical analysis of the effect of hydroxyapatite augmentation for trochanteric femoral fractures.

Hydroxyapatite (HA) augments are used to treat trochanteric femoral fractures. However, the efficacy of HA augmentation has not been fully described in trochanteric femoral fracture surgery. In total, 85 patients were enrolled in the present study; all had trochanteric femoral fractures between January 2016 and October 2020, 45 with HA (HA group) and 40 without HA (N group). The intraoperative lag screw insertion torque was directly measured and the amount of lag screw telescoping with and without HA augmentation after surgery was analyzed. Maximum lag screw insertion torque (max-torque), bone mineral density in the opposite femoral neck (n-BMD), tip apex distance (TAD) of the lag screw, radiographic findings including fracture union, the amounts of lag screw telescoping and occurrence of complications were evaluated. A total of 12 patients were excluded if they were aged under 60 years old, had ipsilateral surgery and disorders in the hip joint, TAD of the lag screw ≥26 mm on postoperative radiographs and had measurement errors. A total of 73 fractures could be analyzed: HA group (n=36) and N group (n=37). Max-torque/n-BMD ratios were higher in the HA group compared with in the N group (7.23±2.71 vs. 5.93±1.91 g/cm2·N·m; P=0.04). The amounts of lag screw telescoping in the HA group were smaller compared with the N group (1.41±2.00 vs. 2.58±2.34; P=0.05). Evaluation of screw insertion torque showed maximum screw insertion torque correlated well with n-BMD in both groups, HA (R=0.57; P<0.01) and N group (R=0.64; P<0.01). No correlation was found between maximum screw insertion torque and TAD in both groups, HA (R=-0.10; P=0.62) and N group (R=0.02; P=0.93). All fractures were radiographically united without any complications. These results support the effectiveness of HA augmentation, indicating higher resistance against rotational instability and reduced lag screw telescoping in trochanteric femoral fracture treatment.

Minimally invasive clamp-assisted reduction and long InterTAN nail fixation for Seinsheimer type V subtrochanteric fractures: a case series describing the technique and results

BackgroundSurgical treatment of Seinsheimer type V subtrochanteric fractures is extremely challenging due to the difficulty of obtaining and maintaining anatomic reduction and effective fixation. The purpose of this study was to describe a surgical technique for minimally invasive clamp-assisted reduction and long InterTAN nail fixation to manage Seinsheimer type V subtrochanteric fractures and report the clinical and radiological results.MethodsA retrospective study was conducted on patients with Seinsheimer type V subtrochanteric fractures between March 2015 and June 2021. A total of 30 patients treated via minimally invasive clamp-assisted reduction, long InterTAN nail fixation and selective augmentation with a cerclage cable were included. The following data were collected and evaluated: patient demographics, operative time, blood loss, reduction quality, tip apex distance (TAD), time to bone union, Harris hip score (HHS), visual analog score (VAS), and complications.ResultsThe mean age of the 30 patients was 64.8 years (range: 36-90 years). The mean operative time was 102.2 min (range: 70-150 min). The mean loss of blood was 318.3 ml (range: 150-600 ml). The reduction quality involved 27 cases of anatomic reduction and 3 cases of satisfactory reduction. The mean TAD was 16.3 mm (range: 8-24 mm). The mean follow-up time was 18.9 months (range: 12-48 months). The mean fracture healing time was 4.5 months (range: 3-8 months). The mean Harris score was 88.2 (range: 71-100), and the VAS score was 0.7 (range: 0-3). Delayed union of the subtrochanteric fracture site occurred in two patients. The limb length discrepancy, which was determined in 3 patients, was < 10 mm. There were no significant complications.ConclusionOur results indicate that minimally invasive clamp-assisted reduction with long InterTAN nail fixation is encouraging for Seinsheimer Type V subtrochanteric fractures, resulting in excellent reduction and fixation. Additionally, this reduction technique is simple, reliable, and effective in reducing and maintaining subtrochanteric fractures, particularly when intertrochanteric fractures are irreducible.

EVALUATION OF CLINICAL AND RADIOLOGICAL OUTCOME OF INTERTROCHANTERIC FRACTURES OF FEMUR TREATED WITH PROXIMAL FEMORAL NAIL : A PROSPECTIVE STUDY

Background: With purpose to evaluate the outcome of proximal femur nail procedure ,we carried out a prospective observational study in which 40 patients, underwent Proximal Femoral nailing for intertrochanteric fracture femur at Govt. R.D.B.P Jaipuria Hospital, Jaipur, Rajasthan. Materials and Methods: Patients were included as per the inclusion and exclusion criteria in the study. All patients were followed up at 4 weeks, 6 weeks, 8 weeks, 12 weeks, 16weeks and 24 weeks and later depending on the fracture healing status. X ray of the operated hip with femur –anteroposterior and lateral views were taken during follow up. Kyle's criteria and Harris hip score was measured at 6 months follow up. Results:The Kyle's criteria was Excellent in 22 patients (55%), Good in 13 patients (32.5%), Fair in 4 patients (10%) and poor in 1 patient (2.5%) Conclusion: proximal femoral nail (PFN) is a good minimally invasive implant for stable and unstable proximal femoral fractures. It's particularly benecial in elderly and osteoporotic population. Adequate fracture reduction is critical in management of the intertrochanteric fractures. Achieving good anatomical reduction and maintaining the tip apex distance less than 25mm will avoid the implant related complications

Controlled Vertical Transfer of Individual Au Atoms Using a Surface Supported Carbon Radical for Atomically Precise Manufacturing

To explore a proof-of-concept for atomically precise manufacturing (APM) using scanning probe microscopy (SPM), first principle theoretical calculations of atom-by-atom transfer from the apex of an SPM tip to an individual radical on a surface-bound organic molecule have been performed. Atom transfer is achieved by spatially controlled motion of a gold terminated tip to the radical. Two molecular tools for SPM-based APM have been designed and investigated, each comprising an adamantane core, a radical end group, and trithiol linkers to enable strong chemisorption on the Au(111) surface: ethynyl-adamantane-trithiol and adamantyl-trithiol. We demonstrate the details of controlled Au atom abstraction during tip approach toward and retraction from the radical species. Upon approach of the tip, the apical Au atom undergoes a transfer toward the carbon radical at a clearly defined threshold separation. This atomic displacement is accompanied by a net energy gain of the system in the range −0.5 to −1.5 eV, depending on the radical structure. In the case of a triangular pyramidal apex model, two tip configurations are possible after the tip atom displacement: (1) an Au atom is abstracted from the tip and bound to the C radical, not bound to the tip base anymore, and (2) apical tip atoms rearrange to form a continuous neck between the tip and radical. In the second case, subsequent tip retraction leads to the same final configuration as the first, with the abstracted Au atom bound to radical carbon atom of the molecular tool. For the less reactive adamantyl-trithiol radical molecular tool, Au atom transfer is less energetically favored, but this has the advantage of avoiding other apex gold atoms from rearrangement.

Prediction of internal fixation failure of femoral trochanteric fracture by external wall morphology

To investigate the relationship between the shape of the lateral wall and the early failure of internal fixation in the fracture of the femoral trochanteric region(FFT). Total 295 patients with femoral trochanteric fracture underwent internal fixation from January 2015 to January 2020 were selected. The patients were divided into two groups according to whether there was early internal fixation failure after surgery, 19 patients in the failure group and 276 patients in the normal group. Gender, affected side, age, AO classification, body mass index(BMI), preoperative hemoglobin, X-ray measurement of lower lateral wall thickness, preoperative internal diseases, intraoperative blood loss, postoperative tip apex distance(TAD), postoperative neck shaft angle, operation time and other data were compared between two groups. The shape of the lateral wall was compared between two groups, and the correlation between the shape of the lateral wall and the early internal fixation failure of femoral trochanteric fracture was analyzed. All patients were followed up for more than 1 year. There was no significant difference between two groups in terms of intraoperative blood loss, operation time, postoperative TAD, and postoperative neck shaft angle(P>0.05). At the latest follow-up, the visual anaglue scale (VAS) of the failure group was higher than that of the normal group(P<0.01), and the Harris score of the failure group was lower than that of normal group(P<0.05). The receiver operator characteristic (ROC) curve between shape of lateral wall and failure of early internal fixation of femoral trochanteric fracture was drawn. The critical value of the midpoint lateral wall thickness was 16.5 mm, and the area under the ROC curve was 0.845;The critical value of average sidewall thickness was 16.5 mm, and the area under ROC curve was 0.838;The critical value of the axial area of the sidewall was 7.5 mm, and the area under the ROC curve was 0.826. The shape of the lateral femoral wall measured by CT could be used as a predictive factor for the early failure of internal fixation of femoral trochanteric fractures. For patients at risk, more reasonable surgical plans and postoperative preventive measures should be developed.

Apex-Confined Plasmonic Tip for High Resolution Tip-Enhanced Raman Spectroscopic Imaging of Carbon Nanotubes.

This paper reports a handy technical scheme to decorate atomic force microscopy (AFM) tips toward tip-enhanced Raman spectroscopy (TERS) applications. The major attraction of these homemade tips lies in that silver decoration can be confined at the apex of commercial tips by the means of an AFM-controlled electrochemical reaction. The reduction of Ag+ occurs in a highly sealed environment to secure the metal coating efficiency. Key factors include silver nitrate solution to provide Ag+, ambient relative humidity and temperature in a humidity cell, electric potential bias, and tip-surface distance. Subsequently, these silver-coated tips are evaluated for TERS measurement of carbon nanotubes (CNTs) so that both morphological and chemical characteristics of CNTs are concurrently obtained. The Raman spectra reveal that our plasmonic tip competently possesses an ∼30-fold local field signal increase and the corresponding TERS image laterally resolves at the single-pixel level.

Fabrication of Ultra-Sharp Tips by Dynamic Chemical Etching Process for Scanning Near-Field Microwave Microscopy.

This work details an effective dynamic chemical etching technique to fabricate ultra-sharp tips for Scanning Near-Field Microwave Microscopy (SNMM). The protruded cylindrical part of the inner conductor in a commercial SMA (Sub Miniature A) coaxial connector is tapered by a dynamic chemical etching process using ferric chloride. The technique is optimized to fabricate ultra-sharp probe tips with controllable shapes and tapered down to have a radius of tip apex around ∼1 μm. The detailed optimization facilitated the fabrication of reproducible high-quality probes suitable for non-contact SNMM operation. A simple analytical model is also presented to better describe the dynamics of the tip formation. The near-field characteristics of the tips are evaluated by finite element method (FEM) based electromagnetic simulations and the performance of the probes has been validated experimentally by means of imaging a metal-dielectric sample using the in-house scanning near-field microwave microscopy system.

Simulations of Subnanometer Scale Image Contrast in Atomic Force Microscopy of Self-Assembled Monolayers in Water.

Achieving high-resolution images using dynamic atomic force microscopy (AFM) requires understanding how chemical and structural features of the surface affect image contrast. This understanding is particularly challenging when imaging samples in water. An initial step is to determine how well-characterized surface features interact with the AFM tip in wet environments. Here, we use molecular dynamics simulations of a model AFM tip apex oscillating in water above self-assembled monolayers (SAMs) with different chain lengths and functional groups. The amplitude response of the tip is characterized across a range of vertical distances and amplitude set points. Then relative image contrast is quantified as the difference of the amplitude response of the tip when it is positioned directly above a SAM functional group vs positioned between two functional groups. Differences in contrast between SAMs with different lengths and functional groups are explained in terms of the vertical deflection of the SAMs due to interactions with the tip and water during dynamic imaging. The knowledge gained from simulations of these simple model systems may ultimately be used to guide selection of imaging parameters for more complex surfaces.

Improving Cephalad Lag Screw Placement in the Femoral Head During Cephalomedullary Nailing Using a Novel Augmented Reality System.

To measure the effect of a novel augmented reality software designed to aid in lag screw placement into the femoral head for cephalomedullary nails. Retrospective cohort study. Single Level I trauma center. Between November 2017 and December 2020, 114 consecutive patients with a hip fracture that underwent repair with a cephalomedullary nail by one of 2 orthopaedic trauma surgeons were reviewed. Fracture classifications included OTA/AO 31-A1, 31-A2, 31-A3, and 31-B3. The first 57 patients underwent fracture repair without the software (control) and the subsequent 57 patients underwent repair with use of the augmented reality software (AR). Tip apex distance (TAD) and femoral head zone (AP: superior, center, inferior; Lateral: anterior, center, posterior) were measured using standardized techniques. The mean TAD was lower for the AR versus control cohort (10.7 ± 2.9 mm vs. 15.4 ± 3.8 mm; P < 0.001). TAD <10 mm for AR versus control: 25 (43.9%) versus 3 (5.3%), P < 0.001. TAD <15 mm for AR versus control: 50 (87.7%) versus 44 (77.2%), P < 0.001. On the AP view, center position was achieved in 50.9% versus 7.0% of cases for the AR versus control cohort, respectively. On the lateral view, center position was achieved in 68.4% versus 12.3% of cases for the AR versus control cohort, respectively. This study suggests that use of the novel augmented reality software for assistance in lag screw positioning within the femoral head improves overall TAD and ability to achieve the center-center position. Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Blade Augmentation in Nailing Proximal Femur Fractures-An Advantage despite Higher Costs?

Proximal femoral fractures occur with increasing incidence, especially in the elderly. Commonly used implants for surgical treatment are cephalomedullary nails. To increase stability, a perforated femoral neck blade can be augmented with cement. The study investigated whether this results in a relevant clinical advantage and justifies the higher cost. This is a single-center retrospective study of 620 patients with proximal femur fractures treated with cephalomedullary nailing. Between January 2016 and December 2020, 207 male and 413 female patients were surgically treated with a proximal femur nail (DePuy Synthes) using a perforated blade and cement augmentation in cases with severe osteoporosis. Primary outcome measures were the rate of cut-out, tip apex distance and the positioning of the blade in the femoral head. Secondary outcome measures were the implant costs and operating times. Of the 620 femoral neck blades, 299 were augmented with cement. A total of six cut-outs were seen in the first 3 months after the operation. There were three in the cement-augmented group (CAB = cement-augmented blade) and three in the conventional group (NCAB = non-cement-augmented blade). There was a significant positive correlation between age and augmentation, with a mean difference of 11 years between the two groups (CAB 85.7 ± 7.9 vs. NCAB 75.3 ± 15.1; p < 0.05). There was no difference in the tip-apex distance (CAB 15.97 vs. 15.69; p = 0.64) or rate of optimal blade positions between the groups (CAB 81.6% vs. NCAB 83.2%; p = 0.341). Operation times were significantly longer in the cemented group (CAB 62.6 21.2 min vs. NCAB 54.1 7.7 min; p < 0.05), and the implant cost nearly doubled due to augmentation. When the principles of anatomic fracture reduction, optimal tip-apex distance and optimal blade position are combined with cement augmentation in cases of severe osteoporosis, a cut-out rate of less than 1% can be achieved. Nevertheless, it should be noted that augmentation remains expensive and prolongs surgery time without definite proof of mechanical superiority.

Identification of OH groups on SrTiO3(100)-(13×13)-R33.7° reconstructed surface by non-contact atomic force microscopy and scanning tunneling microscopy

Atomic resolution imaging of a SrTiO3(100)-(13×13)-R33.7° reconstructed surface using non-contact atomic force microscopy (NC-AFM) and its simultaneous measurement with scanning tunneling microscopy (STM) is presented. Simultaneous STM and NC-AFM imaging reveals three patterns of image contrast depending on the tip apex condition and the relationship between the SrTiO3(100)-(13×13)-R33.7° surface reconstructed structure and the NC-AFM image contrast. The NC-AFM image contrast variation is deduced from the tip apex polarity on the basis of an analysis of two images with opposite contrast. This interpretation is consistent with the results of simultaneous imaging of the SrTiO3(100)-(13×13)-R33.7° surface. Furthermore, the results and interpretation identified an OH group, which is one of the surface defects, and this adsorption site.

DNA Origami Fiducial for Accurate 3D Atomic Force Microscopy Imaging.

Atomic force microscopy (AFM) is a powerful technique for imaging molecules, macromolecular complexes, and nanoparticles with nanometer resolution. However, AFM images are distorted by the shape of the tip used. These distortions can be corrected if the tip shape can be determined by scanning a sample with features sharper than the tip and higher than the object of interest. Here we present a 3D DNA origami structure as fiducial for tip reconstruction and image correction. Our fiducial is stable under a broad range of conditions and has sharp steps at different heights that enable reliable tip reconstruction from as few as ten fiducials. The DNA origami is readily codeposited with biological and nonbiological samples, achieves higher precision for the tip apex than polycrystalline samples, and dramatically improves the accuracy of the lateral dimensions determined from the images. Our fiducial thus enables accurate and precise AFM imaging for a broad range of applications.

Numerical and Physical Modeling of the Effect of the Cone Apex Angle on the Penetration Resistance in Coarse-Grained Soils

The tip shape of penetrometers and piles has an important influence on the soil resistance mobilized during penetration. Blunt or flat tips typically generate greater penetration resistances, which can lead to refusal during in-situ testing and pile driving in the field. Results are presented from numerical and experimental investigations on probes with conical tips of varying apex angles to quantify the effect of the apex angle on the mobilized penetration resistance and associated failure mechanisms. Discrete element modeling (DEM) simulations were performed in unconfined and confined (i.e., stress-controlled) specimens to model shallow and deep penetration conditions, respectively. Centrifuge penetration tests were performed by quasi-statically advancing an instrumented probe to a depth-to-probe-diameter ratio of 16.7. The numerical and experimental results indicate that at shallow depths, the sharper tips mobilize smaller penetration resistances. In deep penetration conditions, the changes in penetration resistance with tip apex angle are less pronounced. Based on the results of the two investigations and values reported in the literature, a relationship characterizing the functional form between tip apex angle and normalized penetration resistance is proposed. A mesoscale analysis of the DEM simulation shows differences in the failure mechanisms induced by sharp and blunt tips: the zone where large particle displacements and stress changes occur is large and located below the tip for penetration with blunt tips, whereas the zone is smaller and located both laterally and below the tip for sharp tips.

ESR-STM on diamagnetic molecule: C60 on graphene

Electron Spin Resonance-Scanning Tunneling Microscopy (ESR-STM) of C60 radical ion on graphene is a first demonstration of ESR-STM on diamagnetic molecules. ESR-STM signal at gaverage=2.0±0.1 was measured in accordance with macroscopic ESR of C60 radical ion. The ESR-STM signal was bias voltage dependent, as it reflects the charge state of the molecule. The signal appears in the bias voltage which enables the ionization of the lowest unoccupied molecular orbital (LUMO) – creation of radical anion, and the highest occupied molecular orbital (HOMO) – creation of a radical cation of the C60 molecule when it deposited on graphene. In parallel, ESR-STM signal at gaverage=1.7±0.1 was ascribed to Tungsten oxide (WO3) at the tip apex. In several experiments, triplet spectrum was observed, and we ascribed their origin to zero-field splitting of doubly ionized C120O-2 dimer, as argued in previous ESR experiments of C60 samples. Second possibility is hyperfine coupling with two 13C nuclei. In addition, we further validate the interference mechanism previously suggested for ESR-STM noise spectroscopy. The ability of ESR-STM to observe ESR of diamagnetic molecules in parallel with observing their electronic structure, provides a general single molecule identification technique.

Intramolecular Force Mapping at Room Temperature.

Acquisition of dense, three-dimensional, force fields with intramolecular resolution via noncontact atomic force microscopy (NC-AFM) has yielded enormous progress in our ability to characterize molecular and two-dimensional materials at the atomic scale. To date, intramolecular force mapping has been performed exclusively at cryogenic temperatures, due to the stability afforded by low temperature operation, and as the carbon monoxide functionalization of the metallic scanning probe tip, normally required for submolecular resolution, is only stable at low temperature. In this paper we show that high-resolution, three-dimensional force mapping of a single organic molecule is possible even at room temperature. The physical limitations of room temperature operation are overcome using semiconducting materials to inhibit molecular diffusion and create robust tip apexes, while challenges due to thermal drift are overcome with atom tracking based feedforward correction. Three-dimensional force maps comparable in spatial and force resolution to those acquired at low temperature are demonstrated, permitting a quantitative analysis of the adsorption induced changes in the geometry of the molecule at the picometer level.