"Characterization of the Photocurrents Generated by the Laser of Atomic Force Microscopes"

Review of Scientific Instruments 87(8), 083703.

Yanfeng Ji, Fei Hui, Yuanyuan Shi, Vanessa Iglesias, David Lewis, Jiebin Niu, Shibing Long, Ming Liu, Alexander Hofer, Werner Frammelsberger, Guenther Benstetter, Andrew Scheuermann, Paul C. McIntyre and Mario Lanza

Photoactive materials play a crucial role in the development of energy storage devices, such as solar, electrochemical cells, and others. Conductive atomic force microscopy (CAFM) is a powerful tool for nanoscale electronic characterization of photoactive materials. It is well known that environmental light can alter the measurements when scanning photoactive samples. For this reason, measuring in a dark environment has been recognized as the standard CAFM process. However, as an optical feedback laser is necessary to acquire topography, the laser used in CAFM can also generate a high photocurrent, even without any bias between the conductive tip and the sample. While the laser-induced current signal perturbation is well known within the CAFM community, the observation of currents generated by the optical feedback laser in absence of bias is still not fully understood and has never been studied in depth.

For the first time, this paper studies and analyzes the photocurrent induced in the photoactive materials by the feedback laser. CAFM measurements were carried out on photoactive samples using six standard optical feedback AFMs of different manufacturers, as well as a Nanonics tuning-fork based feedback AFM (without using a laser).

The results obtained show that the laser induces abundant parasitic photocurrent even without any bias in the other tested optical feedback AFMs. In contrast, the Nanonics MV4000 system based on Tuning Fork feedback does not induce parasitic photocurrent and thus provides a true current map in complete darkness.

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Read the full abstract here 

By combining the best of both worlds that photons and electrons have to offer, polaritons hold much promise for a variety of applications in optoelectronics and nanophotonics such as miniatiruzed circuits for improved information or energy transfer. Polaritons are hybrid or quasi particles that are made up of photons strongly coupled to an electric dipole. There are different kinds of polaritons such an electron-hole pair that form an exciton polariton, which is present in semiconductors, or electrons at a metal surface that create surface plasmon polaritons (SPPs). Exciton polaritons that are stable at ambient conditions are an active area of research interest. A particular group of semiconductor chalcogenide materials was recently identified to have the existence of polaritons under ambient conditions. However, these materials were previously investigated using far-field methods. These materials are important for their potential applications in information technology, bio-sensing and metamaterials.

In this work, a team of researchers led by Prof. Xu of University of Washington use a Nanonics MV 4000 operating in reflection NSOM to study waveguide polaritons in thin <300nm flakes of WSe2 at ambient conditions. Using this setup, they could directly excite and probe polariton modes by imaging their interference fringes in a method termed “scanning polariton interferometry” at different wavelengths to map out the entire polariton dispersion both above and below the excitation energy. In this study, the polaritons were observed to have a wavelength down to 300nm in WSe2 and propagate many microns below the excitation energy. The near-field illumination allowed for the first time direct excitation and real imaging of the exciton polariton without the need for complicated cavity fabrication. Furthermore, by tuning the excitation laser energy it was possible to map the entire polarity dispersion.

Unique apertured and apertureless NSOM protocols with low background

MultiProbe Apertureless NSOM ANSOM 

Link to pdf version of app note

 In standard approaches to apertureless NSOM (ANSOM) the probe is simply a modulated scatterer with gross far-field optical illumination using elements such as lenses or mirrors, which cause large spot sizes of radiation around the scattering probe. 

It has been very chellenging to develop ways to reduce the artifacts that arise from the background that is created by these far-field optical elements; this background can interfere with the desired signal from the nanometric tip of an atomic force sensor capturing the near-field surface component in a large far-field radiation background.

The MultiView 4000 with its unique multiprobe capability enables the reduction of both the optical and mechanical background and thus increase the overall S/N.

One probe can be used as a limited illumination spot onto a second scattering probe.  This both reduces the optical background and generates the correct k vectors to excite the scattering probe.  In addition, since the feedback employs tuning forks, the scattering probe can be kept with an oscillation amplitude of 1 nm.  This has been shown to be critical due to the fact that the probe tip has to be modulated close to the surface without a jump to contact and without variation in z which adds additional background to the signal from mechanical sources.  Furthermore, the rigid tuning fork frequency significantly enhances heterodyne and homodyne lock-in detection schemes to further reduce background.

	This scheme diagrammatically describes the two probe configuration where the illuminating probe excites the scattering probe and this probe can even be a single gold nanoparticle with high scattering contrast at the exposed tip of the AFM probe for increased signal from the very tip of the probe.  These probes have been developed by Nanonics for Tip Enhanced Raman Spectroscopy

Near-Field Plasmon Excitation & Apertureless Scattering and Collection  Apertureless NSOM ANSOM

A Multiview 4000 with a two Probe SPM setup has been used for effective localized illumination of a plasmonic structure with an apertured NSOM probe which produces all k-vectors, and so it is most efficient for such plasmonic propagation. The propagating plasmons are scattered and then collected with a second probe, which has a very low dielectric constant and minimal perturbation of the plasmonic propagation

Two Probe Setup Scheme: An apertured probe to produce an evanescent field with a spectrum of k vectors to effectively excite SPP (Left Probe). Right probe is a very low dielectric contrast, highly exposed, non-interfering scanning and work in Photon Tunneling mode to scatter SPP and directly collect the photons produced by such scattering	MV4000 picture of two probes in close nanometric proximity. MV4000 provides flexible probe and sample piezo scanning stages for fine and coarse probe positioning and scanning. The image above shows two probe attached to Tuning Forks for the ultimate in AFM force sensitivity.	A microscope picture (100x objective) top view shows two tips of apertured (Left) NSOM fiber probe providing 532nm near-field illumination and an Apertureless NSOM probe (Right). Surface plasmons are generated on top of an Au strip and scattered by the scanning ANSOM tip.

Left: AFM Height image of the Au strip performed with the ANSOM scanning tip. The circle at the bottom shows the effect of the illumination apertured tip when scanning in its close proximity. Middle: ANSOM image performed with the scanning tip.  Rich contrast is seen by the apertureless probe doing the AFM and ANSOM imaging. Right: 3D ANSOM image shows sustained plasmon propagation and then rapid decay

 Apertureless Probes  

Standard probes that need to be used in order to effectively scatter the plasmonic signal have significant perturbation on the plasmonic propagation because of the need to use probes with high dielectric constant to obtain effective signal to noise in such scattering experiments. 
Nanonics exclusively provides Apertureless probes of glass with plasmonic or Non-Plasmonic Scattering Particles.  Nanonics ANSOM probes are low dielectric constant, provide non-interfering scanning, highly exposed and work in Photon Tunneling Mode to scatter SPP. For such plasmonic probes, glass provides for high dielectric contrast for exceptional antenna effects at the tip of the probe.  Such probes can be provided with a nanoparticle as small as 10 nm or simple 5nm diameter glass probes.

Apertureless IR NSOM, nanoIR  

In the IR ANSOM regime standard far-field optical elements give large spot sizes of microns to tens of microns which seriously compromise the nature of the signal detected by the nanometric tip of an atomic force sensor.  Applying a Dual Probe system allows for ultra low background with minimal spot illumination size through an infrared fiber probe, which is nanometrically held in close proximity using the dual probe geometry to a scattering low dielectric glass probe or a single  gold nanoparticle probe or a silicon exposed tip probe.  Such a tip is generally modulated in close proximity to a surface in order to delineate the near-field interactions.

NanoIR Probe: Unique methods for IR illumination with a 100 nm point heat source for broad band IR irradiation of a scattering probe tip using multiple probe capabilities and with subsequent interfero-metric IR spectral resolution.  Overcome tens of micron spot sizes with lens based IR optical illumination.

Nanonics MultiProbe Apertureless NSOM: 

• Multiprobe systems are singularly capable of exceptional apertureless and scattering NSOM imaging.

• Ideal Apertureless solution with minimum stray light & maximum plasmonic excitation

• MultiProbe ANSOM appears to have significant potential to reduce background and maximizing signal at the highest resolution

FPN Nanolithography deposition of BSA protein with simultanous AFM imaging 

MultiProbe Nanolithography Writing and Reading

Link to pdf version of this app note

 The images and movie below show deposition of BSA protein lines onto a silicon surface with an MV4000 Two Probe system for simultaneous "writing" and "reading". The size of the deposited protein feature is controlled by the applied forces, which are defined by the setpoint of the AFM tip-sample interaction. The movie shows a fountain pen nanopipette probe delivering the protein to the surface and then retracting from the surface. The lines are then immediately imaged with the second on-line AFM probe of the MultiProbe AFM system

 AFM images of the deposited structure is seen at the end of the movie and in the right image below. The MultiProbe system allows for improved online imaging of the deposited structures without switching to a "writing" probe from an imaging probe, and thus it provides a quick and accurate solution for finding and imaging lithographic patterns.

(Left) BSA protein deposition with FPN capillary nanopipette probe where pipette's tip is indicated by a blue circle. (Middle) AFM imaging with a second online probe where tip is indicated by blue circle. (Right) 3D AFM Height image performed with the AFM probe showing BSA printed lines

 See more information about Nanonics FPN Nanolithography: http://nn.taktiko.co.il/products/chemical-writing

Reading and Writing Movie:

A “point” SPPs source is generated first by an NSOM probe and then a second NSOM probe detects interference patterns

 SPP Interference with MV4000 Two Probe System

The study of Surface Plasmon Polariton (SPP) interference and propagation is increasingly recognized as a very effective way to concentrate and guide light in nanometric domains. A team in the University of Science and Technology of China  demonstrated a unique approach for recording SPP interference from a "point" source on a plasmonic structure utilizing the Nanonics MultiView 4000^TM MultiProbe Scanning Probe Microscope (SPM) system.

MultiView 4000 Two Probe system setup has been used for NSOM excitation of  SPPs with a point source of a 100nm and for NSOM collection of the interference patterns with a second NSOM probe with 100nm aperture.	NSOM images of the SPPs distribution on the sample with excitation position at b, c, d,  and e along the red line shown in a. d shows the numerical calculation

The researchers used two cantilevered 100nm aperture NSOM probes with the MultiView 4000 in order to test the relation between the visibility of interference patterns and the size of the point source. They observed the SPPs interference phenomenon of a ring structure with different point source sizes by using one probe as a point source for near-field excitation of SPPs and the other for near-field collection of the interference patterns

 The patterns were generated through the interference of the propagated and reflected SPPs to and from the ring walls. The intensity distribution of SPPs was measured in tip scanning mode with the sample and the illumination source kept stationary during the measurement.

Published: Appl. Phys. Lett. 98, 201113 (2011)

Click here for more information on the Nanonics MultiView 4000 system

Observing energy transfer and wave guiding in phycocyanin nanowires using a MV4000 Two Probe system

Energy transfer via phycocyanin nanowires has been shown by measuring near-field luminescence with a MV4000 two probe system. One NSOM probe has been used for localized excitation (532 nm light, 150 nm diameter aperture) and a second NSOM tip (150nm diameter aperture) has been used simultaneously to map AFM and Luminescence along the nanowire.
The MV4000 allows for accurate placement of the NSOM illumination probe on top of the nanowire and also allows for simultaneous tip-scanning in close proximity to the illumination tip.

 Published: Phys. Chem. Chem. Phys., 2014. DOI: 10.1039/c4cp00345d  

  Click here for more information on the Nanonics MultiView 4000 system

Avoiding collisions in a multiprobe setup:  Investigation of mechanical interactions between two tips using a MV4000 Two Probe NSOM microscope 

A method to monitor the distance between two tips of tuning fork-based AFM/NSOM systems is described. The distance monitoring is performed by recording the crosstalk signal which characterizes the interaction strength between the two oscillating tuning forks and tips.

The measurements are based on aerodynamic and shear force interactions between the tips.  Both interactions increase with decreasing distance between the tips. The shear force interaction is a short-range effect and it is observed within a region of several tens of nanometers. This interaction leads to a sharp increase in the crosstalk signal once the scanning tip comes into the immediate vicinity of the stationary one. This effect is a reliable tool to detect the mechanical interactions between two tips and thus prevent them from colliding during scanning. (Klein et al)

Published: Appl. Phys. B. DOI 10.1007/s00340-012-5182-7(2012)

Click here for more information on the Nanonics MultiView 4000 system

Multiprobe setup to heat and probe a nanowire's thermal expansion

A MultiProbe setup is used to heat and then thermally probe a nanowire, revealing the thermal distribution within the wire

• MV 4000 Dual Probe Nano-Heater using Thermo-couple probes

• Nanoheater (below, left ) probe was used to locally heat a Nanowire on Nickel pads.

• Thermocouple probe (below, right) was used for AFM/Thermal scanning of the Nanowire and observe local coldspots in the wire

See here for a video showing an example of a multiple nanoheating experiment. 

For Information about Thermal Imaging see: http://www.nanonics.co.il/thermal-spreading-resistance-electrical.html

Click here for more information on the Nanonics MultiView 4000 system

Surface plasmons excitation with STM probe and collection with apertured & apertureless NSOM photon tunneling probes

MV4000 Dual Probe function provides a flexible platform for simultanous STM and NSOM functionality. Such platform is ideal for surface plasmons (SP) excitation with an STM probe and effective imaging of the SP both in aperture NSOM and apertureless via photon tunneling process. 
The following scheme describes the dual probe setup for excitation of surface plasmons on 35nm gold coated surface. The second probe is an NSOM probe use for localized AFM/NSOM imaging of the generated surface plasmons at nanometric close proximity from the STM excitation probe.  

Left: Dual probe Setup for SPP excitation with STM probe and near-field imaging with a second NSOM probe. The multiprobe setup allows for clear optical access from above and below without any obscuration. Right: An optical microscope image from above shows dual probe STM/NSOM in during operation. Cantilevered bent probes allows for clear optical access and for multiprobe operation at close proximity. 

AFM NSOM Imaging of Surface Plasmons excited with STM Probe 

Simultaneous AFM-NSOM imaging of surface plasmons with an apertured NSOM probe. The cantilever NSOM and STM probes geometry enables bringing the NSOM probe to a close proximity from the excitation STM probe and thus allows mapping of the SP the poitn where they are generated.  In addition, the MV 4000 system incorporates sample scanning and tip scanning piezo stages and thus allows for flexible an accurate positioning of both probes until achieving "soft contact" between the probes. Such a procedure of two probe distance adjustment is performed while the two probes arekep in feedback with the sample and thus allows for safe probe manipulation

The image below shows the results of the above protocols obtained by the NSOM probes in probe scanning mode.

Ultra Sensitive Tuning Fork for AFM/STM Feedback

MV4000 multiprobe system uses ultrasensitive Tuning Fork feedback which is ideal for such combined AFM and STM feedback. The tuning fork high force sensitivity and lack of jump-to-contact allows for defined tip-sample interaction down to close proximity from the sample (<1nm) without any force discontinuity (which does occur in optical feedback AFM techniques, during to jump-to-contact and ringing adhesion effects). Such distance stability permits obtaining simultaneous AFM and Tunneling

Unique AFM-STM probes

Nanonics uniquely provides probes which are able to scan with either AFM or STM feedback using the same probe with easy switching between modes

Single and multiple atomic steps imaging with AFM and STM feedback. Line profiles of HOPG steps with AFM feedback (middle) and STM feedback (Right) performed with the same probe.

Nanonics MultiView AFM/STM system has been used for obtaining Work Function information on Q-Dot using Tunneling Feedback.  

Published:  Nano Lett. 2013, 13, 2338−2345

Click here for more information on the Nanonics MultiView 4000 system