Near-field scanning optical microscopy (NSOM) has been by far the most successful among super-resolution optical microscopy techniques in probing and characterizing optical properties of single molecules and nanomaterials [1–8]. The progress in understanding the morphology of Bulk Heterojunction Organic Photovoltaics (BHJ OPVs) with different techniques was reviewed recently . Optoelectronic properties such as light-emission and photocurrent from nanomaterials and devices have been studied with NSOM at optical resolutions beyond the diffraction limit [10–12]. This technique has been utilized to study plasmonic nanoparticles , waveguiding , and also as a lithographic tool [15–17]. NSOM can also be utilized as a powerful tool to obtain local spectroscopic information in the visible-IR range [18–20]. Tip enhanced Raman scattering has been an important tool to study materials using NSOM [21–22]. In addition to probing material properties, there has been considerable effort in designing tips of different geometries and metal coatings that can provide better resolutions or carry out lithography [17, 23–25]. Recently, antenna like tips called ‘campaniles’ have been utilized to excite plasmonic nanoparticles and obtain images with higher resolution [26–27].
One important application of NSOM has been the study of photoconductivity or the generation of current due to incident light. Photoconductivity is particularly a very useful property of materials that constitute the active component in solar cells and photodetectors [12, 28–31]. The magnitude of photocurrent is decided by the photon to charge conversion efficiency of the active absorbing species as well as electrical transport processes, bulk organization/morphology and electrode interfaces. In the past, p-NSOM has been utilized to probe defects and non-uniformities in InGaAsP lasers . However, in the last decade new optoelectronics materials like organic semiconductors and quantum dots have demonstrated appreciable performance in solar cells, displays, detectors and sensors. These materials may be inherently disordered and possess locally non-uniform current generating regions. Mapping and studying local inhomogeneity holds the key to understanding the critical limiting processes and optimizing their macroscopic performance. To this end, NSOM has become a powerful tool to not only map the two or three dimensional microstructure but in some cases to also map the local functionality. p-NSOM has been utilized to image nanostructured materials that have found applications as active materials in solar cells  and lasers . The length scales that have been accessed by p-NSOM cannot be otherwise accessed by conventional optical microscopy techniques. Confocal microscopy has been utilized by several groups to study photoconductivity but this method is limited by optical diffraction and higher resolutions cannot be achieved . Several conjugated polymers like polythiophenes, polyfluorenes  and poly(para-phenylene vinylene) have been studied using NSOM [36–38]. Charge collection pathways, space charge regions and mixed amorphous or crystalline domains could be imaged unprecedented optical resolution using photocurrent NSOM studies. Photoconductivity studies at the near-fields have not been limited to the optical wavelengths and there have been studies in microwave based NSOM to study non-uniformities in silicon solar cells and graphene [39–41]. A high resolution photocurrent microscopy technique based on the cathodoluminescence of quantum dots has been demonstrated . Additionally, biomaterials like membrane proteins have also been studied using NSOM where illumination of light can give rise to local displacement currents [43–52]. The NSOM tip can also be utilized to illuminate a finite size of molecules in a monolayer or few-layer membrane protein film. Finite size characteristic effects appearing as noise features in the transmittance were observed in monolayers of light active bacteriorhodopsin films . These approaches to simultaneously study structure and functionality of membrane proteins should have immense utility in designing and utilizing biophysical systems and in the rapidly upcoming area of optogenetics.
An important factor that makes NSOM a valuable microscopy tool is the existence of domain-length scales of several optical nanostructured materials being comparable to the aperture diameter, e.g., the domain sizes in bulk heterojunction organic solar cells appear over a range of 20–100 nm, which largely falls in suitable length scales for NSOM probing. The dimension of plasmonic nanostructures is typically in a range which allows NSOM to excite the plasmons [54–56], study waveguiding [14, 57] and explore extraordinary optical transmission effects . In scenarios where the actual dimension of the nanostructures is not resolved by NSOM, interfaces between metal and semiconductors can provide crucial information about the nature of charges, charge density and the extent of space charge regions. e.g., semiconductor nanowires have dimensions in this range and the semiconductor-metal interface can be imaged to ascertain whether it an ohmic or Schottky contact [59–60]. Typically, atomic force microscopy (AFM) based conductive modules can be utilized to image conductive properties of metallic, semiconducting and dielectric thin films [61–66]. However, these techniques do not capture the key aspects of the photophysical phenomena in these systems. Optical contrast arising from different types of emission and absorption from different regions in combination with the possibility to study the photocurrent variation from these regions provides a more complete understanding at the microscopic level.
In this review, we present the different modes of p-NSOM technique. We classify major reports into three modes of operation depending on the illumination and device geometry. We highlight important findings and their implications in device physics, spectroscopy and optimum operation. We also point out the fact that even though the term photoconductivity has been popularly utilized, the actual quantity measured is the photocurrent. However, additional experiments can be carried out to arrive at the photoconductivity from the photocurrent data.
2 Near-field optical microscopy
2.1 Theory and operation
A conventional optical microscope utilizes far-field components of the light for imaging. In other words, the sample and the detector are placed at distances that are much larger than the wavelength of the light that is utilized for imaging. In this mode of operation the limit to the resolution is dictated by the Abbe’s diffraction limit given by 0.61λ/(NA), where λ is the wavelength of light and NA is the numerical aperture. e.g., for a wavelength of illumination of 500 nm, the limit of resolution is approximately 250 nm. Hence, the resolution of an imaging system that uses far-field illumination and collection is limited by the wavelength of light. NSOM on the other hand, utilizes the near-field component, which under certain conditions is capable of overcoming this limit as originally proposed by Synge .
In Figure 1, the sample is illuminated from a small aperture with aperture dimensions much lower than the wavelength of light. Additionally, the requirement is that either the illumination or the collection should be very close to the sample-aperture distance, d<<λ, where λ is the wavelength of illumination. If the above two conditions are met, the resolution is limited by the aperture dimensions and not the wavelength of light. The magnitude of the near-field component is strong in the vicinity of the aperture and drops exponentially as a function of axial distance from the aperture.
With the advent of lasers, advanced lithographic techniques, optical fibers and waveguides along with precise piezoelectric translation stages, NSOM has become a popular tool to probe optical properties of materials at unprecedented optical resolutions. Typically, an optical fiber is shaped or tapered in order to obtain tips with diameters in the range of 50–150 nm . The tapered region is generally coated with a metal like gold in order to avoid leakage of optical power [68–70]. Scanning is either carried out using the tip or translational stages equipped with precise piezoelectric-motion based platforms.
2.2 NSOM modes of operation
NSOM can be implemented on conventional optical microscope platforms making it very versatile for different modes of operation. Since, either the sample or the detector needs to be in the near-field regime, there are several geometries that have been demonstrated.
2.2.1 Illumination transmission
In this mode, the tip illuminates a semi-transparent sample on a transparent substrate and the transmitted light is captured using a detector like a photomultiplier tube (PMT) as shown in Figure 2A. This mode can be utilized to study transparent samples like bacteriorhodopsin thin films [38, 53, 71].
In this mode, the tip only collects emitted or transmitted light from the sample as shown in Figure 2B. The illumination may be far-field and the sample may be semi-transparent , e.g., polarization effects in quantum dots have been imaged in this mode . This mode has also been utilized in terahertz imaging .
2.2.3 Illumination collection
In this mode, the tip illuminates as well as collects lightfrom the sample as shown in Figure 2C. This mode is useful for fluorescent samples on reflective substrates. A filter set can be utilized at the detection point to eliminate reflected light at illumination wavelengths. Organic thin films have been imaged using this geometry .
In this mode, the sample is illuminated by the tip but the reflected light may be collected by another detector as shown in Figure 2D, e.g., another NSOM tip in the vicinity. This mode is well suited to study opaque samples, waveguiding in microstructures and plasmonics, e.g., dye doped polymer microspheres have been imaged in this mode .
2.2.5 Reflection collection
In this mode, another light source may be used to excite the sample and the reflected light is collected using the NSOM tip as shown in Figure 2E.
3 Photoconductive NSOM
In the p-NSOM mode the sample to be probed is a working device consisting of a photoconductive material capable of current generation under local illumination. Hence, the illumination, illumination-collection or the reflection modes of NSOM can be employed to carry out p-NSOM. Typically, a near-field probe with an aperture dimension of 50–100 nm is brought into contact (d~10 nm) with the sample. At this tip-sample distance light emerging from the tip predominantly consists of the near-field component that overcomes diffraction effects. If a raster scan is performed the optical features in the range of 50 nm can be resolved. Additionally, at each point the photocurrent can be measured as photogenerated carriers are swept to their respective electrode by either an applied bias or an internal built-in field within the device. Since the size of the light spot is typically 50–100 nm in diameter, the spatial extent current generating regions are comparable to the light spot dimensions. This results in a high resolution photocurrent map in addition to an optical map. Although, high resolution optical and photocurrent maps can be obtained, the effect of near-field itself on the absorption of the sample needs further investigation. It is intuitive that a favourable alignment of the dipoles in the chromophore with the near-field can enhance absorption, a fact that has been demonstrated in bacteriorhodopsin films .
Near-field microscopy requires sensitive optical detectors that can capture optical contrast. Since the optical power delivered is in the range of 5–50 nW , the magnitude of the photocurrent is in the range of pA-nA. Hence, noise-free detection of current along with amplification at various stages is essential. In order to obtain local current generation maps from interfaces in bulk-heterojunction solar cells or lateral bulk heterojunctions (LBHJ), lock-in amplifiers are generally utilized to boost the signal to noise ratio .
The following section presents different modes or geometries of p-NSOM along with important results.
3.1 Modes of photoconductive NSOM
Most of the p-NSOM reports can be broadly classified into three categories: (a) lateral electrode, (b) transparent electrode geometry and (c) electrode periphery geometry as shown in Figure 3. Each mode is suited for certain device architectures and can, in many cases, provide simultaneous information about topography, photocurrent, fluorescence, transmission depending upon the materials being probed.
3.2 Lateral electrode geometry
In this geometry the tip scans or illuminates a photoconductive material with laterally separated electrodes as shown in Figure 3A. A near-field source or a tightly focused laser beam raster scans the sample. Local illumination at each point creates charge carriers that can be collected at their respective lateral electrodes under an applied bias. The efficiency of collection of charge carriers is dependent on the distance they require to drift from the source of generation. When the probe light source is closer to the electrodes, charges can easily drift leading to higher photocurrent magnitudes. As the probe moves farther away from the electrode, the photocurrent magnitude goes through a minimum as the probability of charges being trapped and recombining is high in this regime. Hence, a map of space charge regions can be obtained in this mode which allows access to metal semiconductor interfaces. Typically nanowires in between lateral electrodes , field effect transistors (FET) [78, 79] and lateral BHJs can be imaged in this mode , e.g., Burrato et al carried out near-field photocurrent (NPC) imaging on InGaAsP multi-quantum well lasers and obtained very useful insights into defects and carrier leakage paths . This technique had several advantages over the electron beam induced current (EBIC) method as it could be done in the absence of vacuum and the wavelength of excitation could be tuned.
This method was utilized to probe local photocurrent in CdS nanowires between lateral electrodes. Ti/Al electrodes were evaporated on the CdS nanowires [59, 60] and a frequency doubled Ti:Sapphire laser (λ=400 nm) was used as the excitation source. Under illumination a pronounced photo-response was recorded. If the nanowire is flooded under light, the response was similar to a metal-semiconductor-metal (MSM) detector. In contrast, local illumination showed an asymmetric response with applied bias. If a 50–70 nm tip was utilized to probe the device at different locations. I-V sweep carried out at the electrode edges showed an asymmetric response if the tip illuminated regions close to the forward or reverse biased metal-semiconductor contacts. Hence, it was possible to image regions of high and low photocurrent generation with NSOM on CdS nanowires with lateral electrodes.
3.2.2 Lateral bulk-heterojunction solar cells
The lateral BHJ can be utilized to complement measurements made in vertical solar cell architectures. In this method, instead of conventional sandwiched electrodes, a BHJ is coated with two lateral electrodes [7, 80, 81]. In the case of sandwiched electrodes, the charge collection is in vertical geometry leading to highly efficient devices.
But lateral geometry hinders any access to the individual layers that are buried inside. If the electrodes are deposited in a lateral manner, akin to transistor geometry, access can be gained to the BHJ as shown in Figure 4. However, the distribution of electric field is non-uniform in the lateral heterojunction. Lombardo et al have carried out photocurrent scanning microscopy on LBHJ using a confocal microscope as shown in Figure 5 . It is not strictly a near-field approach, but the principle of utilizing spatial photocurrent to image space charge and other important regions in the devices can be probed. One important observation from these measurements is mapping the space charge region (SCR) close to the electrodes. Additionally, optical measurements like reflection and fluorescence can provide useful information along with the photocurrent scans. The system studied in this report was poly[(4,4′-bis(2-ethylhexyl)dithieno[3,2-b:2′,3′-d]silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl (PSBTBT) and 6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and photocurrent maps were acquired at different bias voltages. The SCR extent was determined to be 5–7 μm as shown in Figure 6. It was concluded that this donor-acceptor combination was well suited for a regular vertical organic solar cell .
Photoconductive AFM is another method that utilizes an inverted confocal microscope to illuminate the sample from the transparent electrode. The scan is carried out using a conductive AFM tip on the polymer side of the device as shown in Figure 7. This technique does not utilize the near-field but the resolution of the image is high as the scanning is carried out by an AFM cantilever. Pingree et al utilized this method to follow the evolution of morphology of (P3HT:PCBM) solar cells under thermal annealing . Analysis of the morphology and the photoconductive AFM were in agreement with macroscopic device parameters, e.g., the evolution of hole current from the photoconductive AFM images was shown to closely follow bulk hole mobility calculated from space charge limited current measurements .
However, a conductive tip is a source of high charge density and whether that affects the sample and device physics is generally ignored.
3.2.3 Transistor geometry
In this mode, the channel region of the working field effect transistor (FET) can be probed under light as shown in Figure 8. It is similar to the lateral electrodes geometry but the substrate here is the gate on which a thin dielectric layer is deposited. The semiconductor is deposited on the dielectric and the electrodes are then thermally evaporated. The channel can be accessed by the NSOM tip. Mueller et al studied graphene transistors using this geometry . They utilized p+-doped silicon with the dielectric as SiO2 and graphene was deposited on the dielectric. Source and drain electrodes comprised of Ti/Pd/Au were deposited by electron beam lithography and electron beam evaporation. An Ar-ion laser was used as the light source. NSOM scans with 150 nm resolution revealed interesting features at the graphene-metal interface.
It was observed that the polarity of the charge carriers at the interfaces of graphene/source and graphene/drain reversed when the voltage was increased from -60 V to +100 V as shown in Figure 9. There also was a shift of the region of polarity .
3.3 Transparent electrode geometry
This geometry for p-NSOM utilizes illumination of the sample through a semitransparent electrode as shown in Figures 3B and 10. Thin films of conjugated polymer like poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and polymer blends have been studied by this technique [12, 29, 30, 84–86]. The advantage of this technique is that it can be utilized to probe a working solar cell. Since the NSOM tip is capable of capturing the height information of the sample, an AFM scan can be simultaneously carried out along with the NSOM.
By this method it was possible to map the phase separation of the blend at high optical resolutions. It was also possible to map out current generating regions by overlaying the AFM and p-NSOM images. If a laser wavelength was chosen such that only the donor MEH-PPV was excited, then charge generation was higher in these regions as compared to the PCBM rich regions.
McNeill et al. also studied the poly(9,9′-dioctylfluorene- co-bis(N,N′-(4,butylphenyl))bis(N,N′-phenyl-1,4-phenylene)diamine) (PFB) and poly(9,9′-dioctylfluorene-co- benzothiadiazole (F8BT) system for phase separation and current generating regions . Important conclusions could be drawn by analyzing the AFM and NPC images measured simultaneously for a 1:1 blend device (inset of Figure 11). The morphology of the films exhibits micron sized phases with height variation of ≈50 nm between phases. The photocurrent image indicates that current is preferentially observed within the low height phase, which has been shown to be PFB-rich . However, current is also generated in the higher phase but with magnitude nearly a quarter of that in the lower phase. The cross-sectional profiles across the phase segregated regions (Figure 11) does not show any enhancement in the photocurrent at the domain boundaries. It was ascertained that they were mixed domains due to photocurrent from both the phases. Riehn et al. studied the PFB:F8BT system by p-NSOM and concluded that regions rich in PFB had a certain F8BT concentration and vice versa . The current contribution from the PFB rich domains was about four times that of an F8BT region confirming that the donor absorption was responsible for the majority of charge carrier generation. But there also was a finite contribution to the overall device current from the absorption of F8BT. This was concluded from measurements which pointed to the fact that the mixing of both the polymers in the domains was asymmetric, i.e., PFB rich phases had a higher concentration of F8BT as compared to the PFB concentration in a F8BT rich phase.
This geometry is versatile and can be utilized to probe a variety of systems. However, effects of light interaction with a metal layer or tip are generally not considered in the image analysis. Although, Riehn et al had solved the Poisson equation with a commercial finite element package in order to arrive at perturbation effects of a metalized near-field at the vicinity of a photoconductive material .
3.4 Electrode periphery geometry
Photocurrent imaging in organic solar cells has shown that a sizable fraction of the photocurrent arises from the peripheral regions around the electrode [28, 38, 89, 90]. Hence scanning the region in the vicinity of the electrode can be utilized to image and study the morphology of the bulk heterojunction as shown in Figure 3C. The utility of this technique for rationalizing device performance in different types of BHJ solar cells has been dealt with extensively in a recent review . In contrast to the transparent electrode geometry mentioned earlier, the photocurrent signal dramatically increases at the vicinity of the electrode. This improves the signal to noise ratio and the quality of images. Hence, this method is well suited for devices which may have opaque electrodes where the transparent electrode geometry cannot be used. However, if the active material is transparent over a desired wavelength range, transmission data from the electrode vicinity can be collected using a PMT along with the NPC and AFM data. This technique is capable of providing three sets of information in one scan: i) AFM, ii) Transmission-NSOM (T-NSOM), iii) Near-field PhotoCurrent (NPC), where the AFM images the surface, T-NSOM images the bulk and the NPC images current generating regions and percolation pathways. In principle, with the combination of T-NSOM and NPC images at different wavelength a three dimensional reconstruction of the polymer morphology can be deduced with some reasonable assumptions .
This method has been very useful in quantifying phase separation by carrying out Fast Fourier Transform (FFT) on the images and analyzing the spatial frequency content. Several polymer and DSSC systems have been probed using this geometry. As shown in Figure 12, the NSOM tip typically scans a region which is at a distance of 5–10 μm from the electrode vicinity where there is a finite photocurrent.
Typically a commercial NSOM system (Nanonics, Multiview 4000) is utilized for p-NSOM with a high NA collection objective below the sample that is coupled to a PMT. Simultaneous map of the AFM, NPC and T-NSOM for poly[(4,40-bis(2-ethylhexyl)dithieno[3,2-b:20,30-d]silole)-2,6-diyl-alt-(4,7-bis(2-thienyl)-2,1,3-benzothiadiazole)-5,50-diyl] (Si-PCPDTBT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6] C71 (PC70BM) BHJ was captured under different annealing conditions as shown in Figure 13A–E. The AFM images show increased r.m.s roughness due to the migration of PCBM to the surface under annealing (Figure 13A, B). The NPC image shows the formation of interconnected networks under annealing (Figure 13C, D) and the NSOM image indicates the formation of an optimum bulk morphology under annealing (Figure 13E, F).
A quantitative measure of the improvement in device morphology can be arrived upon by carrying out an FFT of the images and analyzing spatial frequencies . The shifts in the central frequency along with the width of the distribution are indicating changes in morphology and interconnected networks .
Another important result that emerges is the role of mixed phases in BHJ blends. Mixed phases have been the focus of many recent reports and there has been evidence of their existence from x-ray and neutron scattering measurements [91–93]. Due to the capability of the NSOM to provide high resolution photocurrent images, it has been possible to analyze the interfaces of the donor and acceptors in order to study mixed phases. Figure 14A, B shows NPC images for an amorphous and crystalline blend and the corresponding NPC decay. Clearly, the spatial decay of NPC is rapid for the crystalline case where the miscibility of the donor and acceptor is low (Figure 14C). In contrast the decay of NPC in amorphous systems is gradual due to better intermixing of the components. This is an important result which has critical implications in the macroscopic device parameters and also explains the high efficiencies that have been obtained for amorphous systems in spite of lower charge mobility.
This geometry has also been utilized to study dye sensitized solar cells (DSSC) . A sizable photocurrent is observed in the vicinity of the electrode, thereby making it possible to image the AFM, T-NSOM and the NPC for this system. The solid state-DSSC device consists of a 1.6 µm thick nanoporous TiO2 layer coated on 100 nm compact TiO2 layer. Metal-free indoline based dye D102 was used to sensitize the nonporous TiO2 layer. The hole conducting matrix consisting of spiro-MeOTAD doped with Li[CF3SO2]2 and N(PhBr)3SbCl6 was spin coated from chlorobenzene solution. The top 100 nm Ag cathode was coated using the physical vapor deposition method. The reported power conversion efficiency of these cells was 4.1% and these devices absorb more than 90% of the incident light over a wide range of visible spectrum (440–550 nm) .
Figure 15 shows the morphology [panel (A)] and corresponding short-circuit photocurrent [panel (B)] maps of these cells from when illuminated at the vicinity of 5 µm from the hole collecting electrode. The 543 nm light is incident onto the solid hole transport layer through a NSOM tip of 150 nm aperture. The surface morphology as extracted from the AFM image exhibits rough surface features of about 70 nm, whereas the photocurrent image shows large contrast throughout the surface with maximum of 400 pA. These maps point to the non-uniform distribution of the dye in the TiO2 matrix. Further, these measurements can also provide vital clues in understanding charge transport mechanisms and can complement results obtained from other steady-state and transient measurements on these devices . Additionally, the effect of solid and liquid electrolytes on the transport of charge carriers can be ascertained from p-NSOM measurements.
In conclusion, we have reviewed the various geometries and various interesting systems which can be explored with p-NSOM imaging. Several important material parameters and characteristics can be obtained by locally imaging the photocurrent. Understanding of bulk-collective contributions from broad illumination of large area structures can be understood to a significant extent from examining local variations of photocurrent at submicron length scales. The information that is obtained from this method can complement observations from other microscopy techniques, like conductive AFM and X-ray scattering, to provide a complete description of optoelectronic properties and overall photophysical processes in a variety of systems.
The authors thank Prof. Henry Snaith and Dr. Antonio Abate for providing the DSSC for p-NSOM measurements. The authors acknowledge partial funding from the Department of Atomic Energy, the INDO-UK APEX program, the Department of Science and Technology, Govt. of India.
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