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Article Type: Nanosensor update From: Sensor Review, Volume 31, Issue 3
Sensing electromagnetic radiation and advanced nanofabrication techniques
The first part of this update continues with the theme of this issue and discusses how various nanomaterials and nanostructures are being used to detect electromagnetic radiation. The second part reports on recent developments in nanofabrication techniques.
Nanosensors for detecting electromagnetic radiation
One of the main consequences of reducing the size of semiconductors down to nanometre dimensions is an increase in the energy band gap, leading to visible luminescence, which suggests that these materials could be used as scintillators for detecting ionising radiations. Indeed, the use of semiconductor quantum dots (QDs) as gamma-ray detectors is being studied by researchers from the Lawrence Livermore National Laboratory (LLNL). This is part of broader activity, discussed elsewhere in this issue. Commercially available cadmium selenide/zinc sulphide (CdSe/ZnS) core/shell QDs were embedded into a porous glass matrix to form a nanocomposite and then exposed to a 1 mCi 241Am source. The scintillation output was recorded for three days and corrected for background radiation and a distinctive peak was observed that was attributed to the 59 keV line of 241Am. The Gaussian fit of the peak provided a value of the energy resolution (ΔE/E) of 15 per cent at 59 keV. The scintillation of a standard 1×1 in. NaI (sodium iodide) scintillator crystal was recorded under the same conditions and yielded a ΔE/E value of 30 per cent. (Note: low values of ΔE/E indicate better energy resolution). These figures indicate a factor of two improvement of the un-optimised nanocomposite material over a standard NaI crystal. The average photon output (N) of the material under 59 keV gamma irradiation is estimated to be close to 4,210, while the number of photons generated by an NaI crystal under the same conditions is only 2,600. For higher energies, the projected light output would be about 70,600 photons at 1 MeV, assuming that the QD medium has a linear response, which is a factor 1.75 better than the NaI crystal for which N is typically 40,000. Researchers at Los Alamos National Laboratory are investigating nanoparticulate gamma scintillators. A technique for the large-scale synthesis of nanoparticles (<10 nm) of cerium-doped lanthanum halide (Ce:LaX3, where X=F or Br) has been developed and Ce:LaBr3 is one of the brightest scintillators known. The synthesis of the nanoparticles is scalable to large quantities and the particles can be cast into transparent oleic acid or polymer composites with up to 60 per cent scintillator volume loading. Preliminary experiments show that the Ce:LaF3 oleic acid-based nano-composites exhibit a strong photo-peak when exposed to a 137Cs gamma source.
Quantum dots have been investigated widely as a means of detecting other electromagnetic wavelengths. Notable is the work on IR sensing being conducted by the Centre for Quantum Devices at Northwestern University (NWU). Here, a range of QD-based IR photodetectors and detector arrays, including focal plane arrays are being developed, as discussed in an earlier issue of this journal (Vol. 30, pp. 279-281). Many groups have shown that carbon nanotubes (CNTs) can be used as IR sensors but some intriguing research at Michigan State University is studying how wavelength selectivity might be achieved. As a material’s band-gap governs the wavelength of the electromagnetic radiation that can be detected, modifying the band-gap, so-called “band-gap engineering”, has the potential to alter a material’s response. The group at Michigan have found that a CNT’s band-gap is governed by its radius, so with a multi-walled CNT (MWCNT), stripping layers away reduces the radius and thus alters the band-gap. The group has developed a process that uses feedback control to remove layers from MWCNTs and were able to produce CNTs with different radii and band-gaps which could sense multiple IR wavelengths, notably NIR and MIR, at room temperature.
Returning to QDs, many groups are working on THz detectors based on this technology, reflecting the ever-growing interest in terahertz sensing. In 2010, a group from the Bio-inspired Sensors and Optoelectronics Laboratory in the Department of Electrical Engineering and Computer Science at NWU reported a voltage-tuneable QD photodetector for THz radiation based on intersublevel transitions. The intersublevels are formed by lateral electrical confinement applied on quantum wells and the transitions between them can be strongly tuned by the confinement. The peak detection wavelengths can be tuned from ∼50 to ∼90 μm (6.0 to ∼3.3 THz) with a gate voltage range of −5 to −2 V. The peak detection absorption coefficient is in the order of 103 cm−1 at 77 K and the peak detectivity can reach ∼109 cm2 Hz1/2 W−1. The proposed approach has the advantage of forming highly uniform QD sizes and provides an alternative way to detect THz radiation. Some workers are combing QD principles with CNTs and an example is research at RIKEN, the Institute of Physical and Chemical Research in Japan. Here, a sensing structure has been fabricated from single-walled CNTs, with source, drain and gate electrodes made from titanium/gold films (Figure 1). These CNT-QD devices were immersed in a cryostat and conductivity measurements performed at 1.5 K, with and without THz irradiation. Periodic oscillations of the source-drain current versus gate voltage were observed (Figure 2), indicating that the devices work as single electron transistors. THz irradiation cause the generation of new satellite peaks in the Coulomb blockade regime and the energy spacing between the satellite and original peaks is proportional to the photon energy of the incident THz wave. These observations are clear evidence of photon-assisted tunnelling and mean that the CNT-QD structure can be used as a frequency-tuneable THz detector. The device operates at temperatures up to 7 K, thus eliminating the need for cryogenic cooling. The next aim is to improve the detector’s sensitivity and frequency selectivity. A much more sensitive signal readout could be achieved by capacitively coupling the CNT-QD with a quantum point contact device on a GaAs/AlGaAs heterostructure and frequency selectivity could be improved by using a double-coupled CNT-QD, in which photon-assisted tunnelling takes place as a result of electron transitions between two well defined, discrete levels.
Figure 1 Schematic of QD-CNT THz detector
Figure 2 The source-drain current versus gate voltage with and without THz irradiation for the CNT-QD sensor
Nanomaterials are being studied in the context of UV detection and zinc oxide (ZnO) nanostructures are attracting particular attention. Semiconducting ZnO has a band-gap of ∼3.4 eV and the use of nanowires (NWs) or nanorods may offer advantages due to the increased junction area, enhanced polarisation dependence and improved carrier confinement in one dimension. A group from the National Taiwan University have fabricated a near-UV (NUV, λ=300-400 nm) detector based on a p-n heterojunction consisting of the p-type polymer polyfluorene (PFO) and n-type ZnO nanorods (Figure 3). A thin (∼500 nm) PFO layer was deposited to establish a p-type contact to the n-type ZnO nanorods by spin coating. The PFO acts as the hole transporting layer of the device which has a typical hole mobility about 1.6×10−5 cm2/V s. The current-voltage characteristic of the device demonstrates the typical p-n heterojunction diode behaviour and the quantum efficiencies show a difference of almost three orders of magnitude when illuminated under UV and visible light. The responsivity can reach 0.18 A/W at 300 nm by applying a bias of −2 V, and the group argues that this provides a route to fabricate a low cost NUV photodetector. Research at the Georgia Institute of Technology has involved a device based on functionalised polymers and ZnO “nanobelts” (NBs). It was found that the UV-induced photoconductance in the NBs increased by five orders of magnitude after functionalising, using a polymer that exhibits strong absorption at UV wavelengths. This very large increase is attributed to an electron-hole generation process, assisted by the energy states in the polymer. This study suggests that, by selecting polymers with different UV absorption wavelengths, highly sensitive UV detectors with a large range of detection wavelengths can be fabricated using ZnO NBs. A more recent paper by the group describes a “gigantic” enhancement to the response and reset time of a ZnO NW-based UV sensor. By utilising a Schottky contact instead of an ohmic contact in the device, the UV sensitivity has been improved by four orders of magnitude and the reset time has been drastically reduced from ∼417 to ∼0.8 s. By further surface functionalisation with positive charged poly-(diallydimethylammonium chloride) and negative charged poly(sodium 4-styrenesulfonate) polymers, the reset time has been reduced to ∼20 ms. The fast response and high spectral selectivity combined with high photosensitivity suggest the possibility of using ZnO nanostructures as visible-blind UV sensors for a range of commercial, military and space applications.
Progress in nanofabrication
Developing effective nanofabrication processes is vital if nanosensors and other nanoscale devices are ever to be produced industrially rather than just in the laboratory. Accordingly, all manner of novel nanofabrication techniques are being investigated, including methods of integrating nanomaterials with CMOS processes.
A group from the National Taiwan University has developed a simple nanofabrication technique to enhance the performance of silicon photodetectors. They have used a spin-coated nanosphere lithography method to fabricate nanopatterns on optoelectronic devices such as photodiodes, LEDs and solar cells. The nanopatterns are created by first spin-coating a monolayer of silica nanoparticles in an isopropyl alcohol solvent on the sample and then etching. The silica nanoparticles, with a diameter of 100±10 nm, act as a hard mask during the pattern definition. In the first application studied, the nanoparticles were spin-coated on the surface of an n-type ZnO/p-type silicon photodiode. The patterning step caused a dramatic increase in the acceptance angle from 23° for the planar structure to 50° for the device with the nanosphere coating, at a wavelength of 550 nm. In a second approach, the group fabricated a photodiode with a nanocone profile by depositing n-type ZnO and intrinsic amorphous silicon layers on the nanopatterned p-type silicon substrate. The sensor showed a 36.2 per cent enhancement in photoresponsivity compared to a planar device, with the main absorption peak at a green wavelength that corresponds to amorphous silicon’s absorption.
A nanofabrication technique that achieves three of the “grand challenges in nanofabrication”, namely alteration of the density of a nanoarray, reduction of critical feature sizes and the production of reconfigurable lattice symmetries over large areas and in a massively parallel manner, has been reported by a group from NWU. The technique, termed solvent-assisted nanoscale embossing (SANE), has the potential to produce inexpensive, large-area nanoscale patterns in applications ranging from nanosensor arrays to solar cells, plasmonics and data storage. Presently, electron-beam lithography or focused ion beam milling are used to create patterns at nanometre scales but these techniques are not practical for mass production as they must start from “scratch” each time and the patterns cannot be created over large areas. Moulding, imprint lithography and soft lithography can mass-produce patterns but are limited to the fixed features on the mould or the original master template. SANE appears to overcome these limitations. The group started with a 6 in.-diameter polyurethane master with 180 nm features separated by 400 nm and a poly(dimethylsiloxane) (PDMS) elastomer was cast against this to create PDMS moulds. These were first wet with solvent and then placed in contact with a photoresist-coated substrate. To create patterns with higher array densities, a heated photoresist is placed on shrink film in an oven at 115°C and after 20 min, the thermoplastic substrate shrank by a third and the lattice spacing decreased by this amount. Heating the substrate for 40 min reduces the size of the shrink film further, by 50 per cent, and the array spacing changed by a similar amount. This produces a master with an array density three-times higher than that of the original. To create arrays with lower densities than the original, the substrate was simply stretched after heating. Uniformly stretching the film in two perpendicular directions produces a master with spacings that are up to twice as large as the original pattern’s and array densities that are 75 per cent lower. Recently, SANE has been used to create plasmonic nanoparticle arrays with continuously variable separations and hence different optical properties on the same substrate (Figures 3 and 4).
Figure 3 (a) Response spectrum of the ZnO nanowire UV sensor as a function of the wavelength of the incident light; upper inset: optical image of the sensor; lower inset: schematic of the device; (b) I-V characteristics of a sensor in the dark and under 365 nm UV illumination; (c) time dependence of the photocurrent growth and decay under periodic illumination at 365 nm at a bias of 1 V; (d) experimental and fitted curves of the photocurrent decay process
Figure 4 The SANE technique
Creating nanoscale devices with processes that are compatible with standard silicon CMOS poses a significant technological challenge that must be resolved, as today’s massive silicon electronics infrastructure is extremely difficult to change for both economic and technological reasons. Combining III-V semiconductors with silicon is highly desirable as they can be used in sensors, lasers and other optoelectronic devices. The problem is that there is a crystal lattice mismatch between III-V materials and silicon and further, the growth of III-V semiconductors has traditionally involved high temperatures, around 700°C or more that would destroy silicon electronics. An example of such a process is the recently reported work by NIST which has succeeded in producing large arrays of gallium nitride NWs but which involved processing temperatures of over 800°C. However, workers at the University of California, Berkeley (UCB) have succeeded in growing nanopillars of indium gallium arsenide (InGaAs) onto a silicon surface at the far lower temperature of 400°C with metal-organic chemical vapour deposition (MOCVD). It was shown that the nanopillar could generate NIR laser light at about 950 nm at room temperature. The hexagonal geometry dictated by the crystal structure of the nanopillars creates a new and efficient light-trapping optical cavity whereby light circulates up and down the structure in a helical fashion (Figure 5) and is amplified by this optical feedback mechanism. Thus, for the first time, the group has succeeded in integrating an array of III-V semiconductor-based nanolasers onto a silicon substrate using a bottom-up, CMOS-compatible process. In addition, the group has fabricated two other nanoneedle-based devices: GaAs avalanche photodiodes and InGaAs/GaAs LEDs. This technique is potentially well suited to mass-production as MOCVD is already used commercially to fabricate LEDs and thin-film solar cells. In addition to silicon, III-V materials can be produced on sapphire and other substrates and the nanoneedles can be layered and doped, as well as selectively etched, to create robust on-chip structures for applications such as sensors, optical interconnects, field emission devices, nonlinear optical signal generators, displays and nanofluidics. Figure 6 shows a schematic of a nanopillar laser and a series of scanning electron microscope (SEM) images. Workers from the Korea Advanced Institute of Science and Technology, Hewlett-Packard Laboratories and the Sensor and Actuator Centre at UCB have developed a top-down, CMOS-compatible technique for producing a silicon nanowire (SNW) array-based sensor for monitoring pH. Most previous research involving SNWs has adopted a bottom-up approach, using techniques such as the deposition of a liquid suspension of multiple NWs on pre-fabricated electrodes, microcontact printing, optical trapping and MEMS tweezers. However, these approaches do not guarantee the reproducibility and robustness of the mechanical and electrical contacts. The pH sensors were fabricated by a top-down method using electron beam lithography for the nanopatterning of SNW features and reactive ion etching for the pattern transfer to the thin silicon device layer on the silicon-on-insulator substrate. Full details of the process are given in Park et al. (2010), Nanotechnology, 21. It is fully compatible with CMOS, ensures accurate alignment with other electrical components and allows flexible designs of the NW geometry and good control of the electrical characteristics. The sensor showed a large operating range (pH 4-10) and an average sensitivity (ΔR/R/pH) of 2.6 per cent/pH.
Figure 5 The unique structure of the nanopillars strongly confines light in a minute volume to enable sub-wavelength nanolasers
Figure 6 Schematic (left) and scanning electron microscope images of nanolasers grown directly on a silicon surface (right)
A final example of research which unites the two themes of this update – sensing electromagnetic radiation and advanced nanofabrication – is being conducted at UCB and Lawrence Berkeley National Laboratory. In contast to the above, this uses nanoelectronics rather than aiming to achieve COMS compatibility. The research involves the fabrication of highly ordered and parallel arrays of optically-active CdSe NWs and high mobility Ge/Si NWs to create an integrated, NW-based photodetector. These two nanostructures act as photodiodes and transistors, respectively. The fully integrated device consists of three types of active element: the optical nanosensors (photodiodes), based on either a single or parallel arrays of multiple CdSe NWs; a high resistance FET, based on parallel arrays of 1-5 Ge/Si core/shell NWs and a low-resistance buffer FET with the channel consisting of parallel arrays of ∼2,000 NWs. The group fabricated large arrays (13×20 element) and measured the photoresponse of each individual circuit element. Rather than single CdSe NWs, parallel arrays of 5-10 were used as the active element of each sensor to enhance the yield and reduce the variation by taking advantage of the averaging effect. It was found that ∼80 per cent of the circuits demonstrated photoresponsive operation, showing a mean photocurrent of ∼420 μA. Five orders of current amplification was achieved by the NW circuitry. The multi-stage fabrication process was somewhat complex and although specialised methods were use for the NW growth, more routine photolithographic, etching and metalisation techniques were used elsewhere. This work demonstrates that fully intgrated sensing and signal conditioning devices can be fabricated entirely from nanomaterials and as such represent a major technological milestone.
Lee, M.H., Huntington, M.D., Zhou, W., Yang, J.-C. and Odom, T.W. (2011), “Programmable soft lithography: solvent-assisted nanoscale embossing”, Nano Letters, Vol. 11 No. 2, pp. 311–5
Zhou, J., Gu, Y.D., Hu, Y.F., Mai, W.J., Yeh, P.H., Bao, G., Sood, A.K., Polla, D.L. and Wang, Z.L. (2009), “Gigantic enhancement in response and reset time of ZnO UV nanosensor by utilizing Schottky contact and surface functionalization”, Applied Physics Letters, Vol. 94 No. 19