Near-field microscopy with a scanning nitrogen-vacancy color center in a diamond nanocrystal: A brief review

We review our recent developments of near-field scanning optical microscopy (NSOM) that uses an active tip made of a single fluorescent nanodiamond (ND) grafted onto the apex of a substrate fiber tip. The ND hosting a limited number of nitrogen-vacancy (NV) color centers, such a tip is a scanning quantum source of light. The method for preparing the ND-based tips and their basic properties are summarized. Then we discuss theoretically the concept of spatial resolution that is achievable in this special NSOM configuration and find it to be only limited by the scan height over the imaged system, in contrast with the standard aperture-tip NSOM whose resolution depends critically on both the scan height and aperture diameter. Finally, we describe a scheme we have introduced recently for high-resolution imaging of nanoplasmonic structures with ND-based tips that is capable of approaching the ultimate resolution anticipated by theory.     

A multi-inversion-recovery magnetic resonance fingerprinting for multi-compartment water mapping

PurposeThis study aimed at introducing short-T1/T2 compartment to MR fingerprinting (MRF) at 3 T. Water that is bound to myelin macromolecules have significantly shorter T1 and T2 than free water and can be distinguished from free water by multi-compartment analysis.MethodsWe developed a new multi-inversion-recovery (mIR) water mapping-MRF based on an unbalanced steady-state coherent sequence (FISP). mIR pulses with an interval of 400 or 500 repetition times (TRs) were inserted into the conventional FISP MRF sequence. Data from our proposed mIR MRF was used to quantify different compartments, including myelin water, gray matter free water, and white matter free water, of brain water by virtue of the iterative non-negative least square (NNLS) with reweighting. Three healthy volunteers were scanned with mIR MRF on a clinical 3 T MRI.ResultsUsing an extended phase graph simulation, we found that our proposed mIR scheme with four IR pulses allowed differentiation between short and long T1/T2 components. For in vivo experiments, we achieved the quantification of myelin water, gray matter water, and white matter water at an image resolution of 1.17 × 1.17 × 5 mm³/pixel. As compared to the conventional MRF technique with single IR, our proposed mIR improved the detection of myelin water content. In addition, mIR MRF using spiral-in/out trajectory provided a higher signal level compared with that with spiral-out trajectory. Myelin water quantification using mIR MRF with 4 IR and 5 IR pulses were qualitatively similar. Meanwhile, 5 IR MRF showed fewer artifacts in myelin water detection.ConclusionWe developed a new mIR MRF sequence for the rapid quantification of brain water compartments.     

Polymer tips for application of atomic force microscopy

A hydrophobic polymer tip for atomic force microscopy has been fabricated by two-photon adsorbed photopolymerization techniques and has been applied for the high resolution imaging of a hydrophilic metal surface. Using optimized photopolymerization conditions, we have succeeded in fabricating sub-100-nm sized polymer tips. This fabricating resolution of two-photon adsorbed photopolymerization is also confirmed by other supporting experiments. The imaging results show that the capillary-force-induced image distortion can be successfully removed by applying a pure hydrophobic polymer tip with a lateral resolution better than 5 nm, which is difficult to achieve with a commercial tip without any environmental control.     

Receding contact lines: From sliding drops to immersion lithography

Instabilities of receding contact lines often occur through the formation of a corner with a very sharp tip. These dewetting structures also appear in the technology of Immersion Lithography, where water is put between the lens and the silicon wafer to increase the optical resolution. In this paper we aim to compare corners appearing in Immersion Lithography to those at the tail of gravity driven-drops sliding down an incline. We use high speed recordings to measure the dynamic contact angle and the sharpness of the corner, for varying contact line velocity. It is found that these quantities behave very similarly for Immersion Lithography and drops on an incline. In addition, the results agree well with predictions by a lubrication model for cornered contact lines, hinting at a generic structure of dewetting corners.     

The seasonal variation of column abundance of atmospheric CH4 and precipitable water derived from ground-based IR solar spectra

Infrared (IR) solar spectra on clear days were measured automatically by an IR solar spectrometer (ISS), capable of 0.4 cm⁻¹ resolution, developed by us. A line-by-line computation method was used to calculate theoretical atmospheric absorption. In the wavelength range of 3.410–3.438 μm, the absorption is mainly due to atmospheric methane and water vapor. Column atmospheric methane was retrieved from the recorded IR solar spectra. The seasonal variation of column atmospheric methane in Hefei was obtained from the measuremental data of nearly 18 months since April 1997, and the seasonal variation was found to be similar to that of background data. The cycles of precipitable water were also obtained simultaneously, and the observed precipitable water coincided with that of a radiosonde. The instruments, principles of measurement and some of the results are introduced, and the results are also discussed briefly in this paper.     

Quantitative atomic force microscopy with carbon monoxide terminated tips

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes.     

Resolution enhancement in a reflection mode near-field optical microscope by second-harmonic modulation signals

The motion of the probe tip in a near-field scanning optical microscope, dithered by vibration of a tuning fork, can modulate the reflection signal from the sample surface not only at the fundamental dithering frequency but also at its second harmonic. By lock-in amplification of these modulated signals, enhanced optical images are obtained, even with an uncoated fiber probe. In particular, accurate optical images with higher resolution are obtained when the second-harmonic signal is detected, which results from the parametric modulation of the tip-sample separation at the double frequency of the horizontal dithering motion of the tip. Using a DVD ROM with a track pitch of 0.74 mum as a test sample, we observed that the sharp edges around the pits are clearly resolved with the second-harmonic signals and obtained enhanced resolution of ~70 nm full width at half-maximum.     

Near-field microscopy with a single-photon point-like emitter: Resolution versus the aperture tip?

We discuss theoretically the concept of spatial resolution in near-field scanning optical microscopy (NSOM) in light of a recent work [Opt. Express 17 (2009) 19969] which reported on the achievement of active tips made of a single ultrasmall fluorescent nanodiamond grafted onto the apex of a substrate tip and on their validation in NSOM imaging. Since fluorescent nanodiamonds tend to decrease steadily in size, we assimilate a nanodiamond-based tip to a point-like single photon source and compare its ultimate resolution with that offered by standard metal-coated aperture NSOM tips. We demonstrate both classically and quantum mechanically that NSOM based on a point-like tip has a resolving power that is only limited by the scan height over the imaged system whereas the aperture-tip resolution depends critically on both the scan height and aperture diameter. This is a consequence of the complex distribution of the electromagnetic field around the aperture that tends to artificially duplicate the imaged objects. We show that the point-like tip does not suffer from this “squint” and that it rapidly approaches its ultimate resolution in the near-field as soon as its scan height falls below the distance between the two nano-objects to be resolved.     

Non-Contact to Contact Transition： Direct Measurements of Interaction Forces between a Solid Probe and a Planar Air-Water Interface

The interaction force between a solid probe and a planar air-water interface is measured by using an atomic force microscope. It is demonstrated that during the approach of the probe to the air-water interface, the force curves decline all the time due to the van der Waals attraction and induces a stable profile of water surface raised. When the tip approaches very close to the water surface, force curves jump suddenly, reflecting the complex behaviour of the unstable water surface. With a theoretical analysis we conclude that before the tip touches water surface, two water profiles appear, one stable and the other unstable. Then, with further approaching, the tip touches water surface and the non-contact to contact transition occurs.     

Ultraviolet tip‐enhanced nanoscale Raman imaging

We have constructed an ultraviolet (UV)‐apertureless near‐field scanning optical microscope‐Raman spectroscopy system by using an aluminum tip for the simultaneous measurement of topography and Raman scattering of nanomaterials with high spatial resolution. The topography, Rayleigh scattering image, and tip‐enhanced Raman scattering image of the carbon nanotube film showed that a spatial resolution of around 19 nm was achieved. This spatial resolution of UV‐Raman mapping image exceeds that of previous approaches, which have several hundred nanometers of spatial resolution. Copyright © 2012 John Wiley & Sons, Ltd.     

Projection microscopy of photoionization processes in gases

We demonstrate a method combining laser ionization of molecules with projection technique and allowing observation of photoionization processes in gases with sub-focal spatial micro-resolution. A bunch of molecular ions created by the nonlinear photoionization of the imaging gas near a tip extends in a divergent electrostatic field producing a magnifying image on the detector. It can be used to observe the profile of the sharply-focused intense laser beam in a wide spectral range. In proof of principle experiment the water molecules are ionized in ~40-μm laser focal spot in the vicinity of the silver needle with a curvature radius 0.5?mm and the resultant ions are counted by a position-sensitive scheme. According to our estimations, ~1.5-μm spatial resolution has been reached. Using a sharp tip, the spatial resolution can be improved to the sub-micrometer scale and such approach can be applied for short wavelength beam diagnostics.     

A new beamline for infrared microscopy in the SR center of Ritsumeikan University

A new beamline for an infrared (IR) microspectroscopy is under construction in the SR center of Ritsumeikan University. We designed an optical system which collects synchrotron radiation (SR) photons by installing one toroidal mirror and two plane mirrors in the storage ring chamber. As a result, the acceptance angle can be widened up to 250 mrad in horizontal and 63 mrad in vertical. Our aim is to develop an IR microspectroscopy beamline with spatial resolution of the order of sub micron from mid-IR to far-IR. In this paper, we present a designed optical system of IR microspectroscopy and the results of ray-tracing.     

Cloaked near-field scanning optical microscope tip for noninvasive near-field imaging

Near-field imaging is a well-established technique in biomedical measurements, since closer to the detail of interest it is possible to resolve subwavelength details otherwise unresolved by regular lenses. A near-field scanning optical microscope (NSOM) tip may indeed overcome the resolution limits of far-field optics, but its proximity inherently perturbs the measurement. Here, we apply the recent concept of a "cloaked sensor" to an NSOM device in collection mode, showing theoretically how a proper plasmonic cover applied to an NSOM tip may drastically improve its overall measurement capabilities.     

Electroless Nickel Plating under Continuous Ultrasonic Irradiation to Fabricate a Near-Field Probe Whose Metal Coat Decreases in Thickness toward the Tip

This paper describes a size-dependent electroless plating method to fabricate a new type of probe with a locally decreasing thickness of metal and a tiny tip size for a combined high resolution shear-force and near-field optical microscope. In this method, the tip size and decreasing thickness profile, which affect the resolution capabilities of the microscopes, are controlled by adding a continuous ultrasonic wave with a frequency of 1 MHz to a nickel plating bath. The probe with a tip radius of curvature of 25 nm was successfully fabricated at an ultrasonic power density of 1.6 W cm⁻², its metal thickness gradually decreased from 850 to 20 nm toward the distal tip.     

A Method to Obtain a Good Infrared Spectrum of the Wet Acetone Gas Sample without Any Drying PretreatmentSCIEI北大核心CSCD

Water vapor in the beam of a Fourier transform infrared (FTIR) spectrometer or/and in a gas cell is a major source of interference in the infrared measurement of a gas sample. In general, in order to eliminate the effect of water vapor, we have to completely remove these molecules before the measurement of any spectrum. Herein, we provide an approach to collect an IR spectrum of a gas (wet acetone) sample free from water vapor interference without any drying pretreatment. After dozens of scans, the air atmosphere in the sample compartment of the FTIR spectrometer will be slightly adjusted by small amounts of dry air or wet air (room air) depending on the property of water vapor lines (absorbance> 0 or absorbance <0). Water vapor lines will then gradually disappear when the ensuing collection process is continued. The experiment results demonstrated that this method is a facile and very effective way to remove water vapor interference. Moreover, this method works still well for the measurement at 0.5 cm(-1) resolution, typically used in the gas infrared measurement.     

不需干燥预处理过程获得含水汽的丙酮气体样品的高质量红外光谱

测量气体样品的红外光谱时,红外光谱仪器内的水汽和气体样品中所含的水汽会严重干扰气体的红外光谱。为消除水汽的干扰,通常作法是测量前必须将水分子(气态)彻底排除掉,也就是使光谱仪器和气体样品保持干燥状态。该研究提供一个新测量方法,不需任何干燥预处理过程,直接获得含水汽的丙酮气体的红外光谱。经过几十次光谱扫描累加后,观察并根据水汽谱的吸收峰性质(吸光率>0或吸光率<0),向光谱仪样品室引入少量干燥氮气或少量潮湿空气,在接下来的光谱扫描累加过程中,水汽吸收峰将逐渐变小直至消失。实验结果表明该方法去除水汽干扰峰简单高效。新方法在0.5 cm-1光谱分辨率时效果非常好,为气体的高分辨分析提供了有力工具。     

Spaser spectroscopy with subwavelength spatial resolution

We propose a method for high-sensitivity subwavelength spectromicroscopy based on the usage of a spaser (plasmonic nanolaser) in the form of a scanning probe microscope tip. The high spatial resolution is defined by plasmon localization at the tip, as is the case for apertureless scanning near-field optical microscopy. In contrast to the latter method, we suggest using radiationless plasmon pumping with quantum dots instead of irradiation with an external laser beam. Due to absorption at the transition frequencies of neighboring nano-objects (molecules or clusters), dips appear in the plasmon generation spectrum. The highest sensitivity is achieved near the generation threshold.     

Understanding magnetic force microscopy

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.     

Theoretical simulation of Kelvin probe force microscopy for Si surfaces by taking account of chemical forces

A new method of theoretical simulation for Kelvin probe force microscopy (KPFM) imaging on semiconductor or metal samples is proposed. The method is based on a partitioned real space (PR) density functional based tight binding (DFTB) calculation of the electronic states to determine the multi-pole electro-static force, which is augmented with the chemical force obtained by a perturbation treatment of the orbital hybridization. With the PR-DFTB method, the change of the total energy is calculated together with the induced charge distribution in the tip and the sample by their approach under an applied bias voltage, and the KPFM images, namely the patterns of local contact potential difference (LCPD) distribution, are obtained with the minimum condition of the interaction force. However, since the interaction force is due to electro-static multi-poles, the spatial resolution of the KPFM images obtained by PR-DFTB is limited to the nano-scale range and an atom-scale resolution cannot be attained. By introducing an additional chemical force, i.e., the force due to the orbital hybridization, we succeeded in reproducing atom-scale resolution of KPFM images. Case studies are performed for clean and impurity embedded Si surfaces with Si tip models.     

Imitation of non-contact mode while scanning in the presence of an electric double layer?

Non-contact atomic force microscopy (NCAFM) minimizes the physical interaction between the AFM tip and the surface of interest. Several recent studies have reported observation of single atom defects using this technique. The repulsive force is presumably the primary interatomic force (cf. our paper on pseudo-non-contact mode in this issue) responsible for the reported atomic resolution in these studies. The combination of these factors, minimal tip–sample deformation and repulsive force interaction, are responsible for the observation of the single atom defects. In the present study, we show that similar resolution can be achieved utilizing the same two factors but which employs scanning in a surfactant. The method decreases the tip–sample interaction by eliminating the attractive forces between the tip and sample. The surfactant solution induces an electrical double-layer (EDL) on the surface of the tip and sample. This EDL creates additional repulsion that is distributed over a large area, and hence does not contribute noticeably to the image contrast during scanning. However, it does compensate for the high pressures normally experienced by the tip in the absence of surfactant. In addition, the presence of the EDL enhances tip stability during the image scan. This method has been tested on surfaces of such minerals as mica, chlorite, and anhydrite.