Life cycle assessment of emerging technologies at the lab scale: The case of nanowire‐based solar cells

Nanomaterials are expected to play an important role in the development of sustainable products. The use of nanomaterials in solar cells has the potential to increase their conversion efficiency. In this study, we performed a life cycle assessment (LCA) for an emerging nanowire‐based solar technology. Two lab‐scale manufacturing routes for the production of nanowire‐based solar cells have been compared—the direct growth of GaInP nanowires on silicon substrate and the growth of InP nanowires on native substrate, peel off, and transfer to silicon substrate. The analysis revealed critical raw materials and processes of the current lab‐scale manufacturing routes such as the use of trifluoromethane (CHF₃), gold, and an InP wafer and a stamp, which are used and discarded. The environmental performance of the two production routes under different scenarios has been assessed. The scenarios include the use of an alternative process to reduce the gold requirements—electroplating instead of metallization, recovery of gold, and reuse of the InP wafer and the stamp. A number of suggestions, based on the LCA results—including minimization of the use of gold and further exploration for upscaling of the electroplating process, the increase in the lifetimes of the wafer and the stamp, and the use of fluorine‐free etching materials—have been communicated to the researchers in order to improve the environmental performance of the technology. Finally, the usefulness and limitations of lab‐scale LCA as a tool to guide the sustainable development of emerging technologies are discussed.     

Quantitative measurements of C-reactive protein using silicon nanowire arrays

A silicon nanowire-based sensor for biological application showed highly desirable electrical responses to either pH changes or receptor-ligand interactions such as protein disease markers, viruses, and DNA hybridization. Furthermore, because the silicon nanowire can display results in real-time, it may possess superior characteristics for biosensing than those demonstrated in previously studied methods. However, despite its promising potential and advantages, certain process-related limitations of the device, due to its size and material characteristics, need to be addressed. In this article, we suggest possible solutions. We fabricated silicon nanowire using a top-down and low cost micromachining method, and evaluate the sensing of molecules after transfer and surface modifications. Our newly designed method can be used to attach highly ordered nanowires to various substrates, to form a nanowire array device, which needs to follow a series of repetitive steps in conventional fabrication technology based on a vapor-liquid-solid (VLS) method. For evaluation, we demonstrated that our newly fabricated silicon nanowire arrays could detect pH changes as well as streptavidin-biotin binding events. As well as the initial proof-of-principle studies, C-reactive protein binding was measured: electrical signals were changed in a linear fashion with the concentration (1 fM to 1 nM) in PBS containing 1.37 mM of salts. Finally, to address the effects of Debye length, silicon nanowires coupled with antigen proteins underwent electrical signal changes as the salt concentration changed.     

Top-Down Nanofabrication and Characterization of 20 nm Silicon Nanowires for Biosensing Applications

A top-down nanofabrication approach is used to develop silicon nanowires from silicon-on-insulator (SOI) wafers and involves direct-write electron beam lithography (EBL), inductively coupled plasma-reactive ion etching (ICP-RIE) and a size reduction process. To achieve nanometer scale size, the crucial factors contributing to the EBL and size reduction processes are highlighted. The resulting silicon nanowires, which are 20 nm in width and 30 nm in height (with a triangular shape) and have a straight structure over the length of 400 μm, are fabricated precisely at the designed location on the device. The device is applied in biomolecule detection based on the changes in drain current (I_ds), electrical resistance and conductance of the silicon nanowires upon hybridization to complementary target deoxyribonucleic acid (DNA). In this context, the scaled-down device exhibited superior performances in terms of good specificity and high sensitivity, with a limit of detection (LOD) of 10 fM, enables for efficient label-free, direct and higher-accuracy DNA molecules detection. Thus, this silicon nanowire can be used as an improved transducer and serves as novel biosensor for future biomedical diagnostic applications.     

Towards the silicon nanowire-based sensor for intracellular biochemical detection

A microneedle sensor platform with integrated silicon nanowire tip was developed for intracellular biochemical detection. Because of the virtue of miniaturized size and high sensitivity, this sensor has a great potential for studying individual cell or localized bioenvironment by revealing the pH level and/or enzyme activities. The fabrication of the microneedle sensor was primarily based on conventional silicon processing, where a silicon-on-insulator (SOI) wafer with 50 nm thick (100) p-type Si device layer was used as the substrate. The silicon nanowires of 50 nm height and 50-100 nm width were created by electron beam (E-beam) lithography on the tip of microneedle with good electrical connection to the contact pads for convenient electrical measurement. A three layer structure with base, support cantilever, and needle tip was designed to ensure convenient handling of sensors and minimize the invasive penetration into biological cells. In this paper, we demonstrate a preliminary assessment of this novel intracellular sensor with electrical conductance measurement under different pH levels. It is expected that this sensor with proper chemical modification will enable localized biochemical sensing within biological cells, such as neurotransmitter activities during the synaptic communication between neuron cells.     

Analysis of Contact Interfaces for Single GaN Nanowire Devices

Single GaN nanowire (NW) devices fabricated on SiO₂ can exhibit a strong degradation after annealing due to the occurrence of void formation at the contact/SiO₂ interface. This void formation can cause cracking and delamination of the metal film, which can increase the resistance or lead to a complete failure of the NW device. In order to address issues associated with void formation, a technique was developed that removes Ni/Au contact metal films from the substrates to allow for the examination and characterization of the contact/substrate and contact/NW interfaces of single GaN NW devices. This procedure determines the degree of adhesion of the contact films to the substrate and NWs and allows for the characterization of the morphology and composition of the contact interface with the substrate and nanowires. This technique is also useful for assessing the amount of residual contamination that remains from the NW suspension and from photolithographic processes on the NW-SiO₂ surface prior to metal deposition. The detailed steps of this procedure are presented for the removal of annealed Ni/Au contacts to Mg-doped GaN NWs on a SiO₂ substrate.     

Contact Doping with Sub‐Monolayers of Strong Polyelectrolytes for Organic Photovoltaics

Barriers to charge transfer at electrode‐semiconductor contacts are ubiquitous and limit the applicability of organic semiconductors in electronic devices. Molecular or ionic doping near contacts can alleviate charge injection or extraction problems by enabling charge tunneling through contact barriers, but the soft nature of organic materials allows for small molecule dopants to diffuse and migrate, degrading the performance of the device and limiting effective interfacial doping. Here, it is demonstrated that contact doping in organic electronics is possible through ionic polymer dopants, which resist diffusion or migration due to their large size. Sub‐monolayer deposition of non‐conjugated strong polyelectrolytes, e.g., sulfonated poly(sulfone)s, at the anode‐semiconductor interface of organic photovoltaics enables efficient hole extraction at the anode. The performance of contact‐doped organic photo­voltaics nearly matches the performance of devices composed of traditional hole transport layers such as poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). The degree of sulfonation of the dopant polymer and the thickness of the ionic dopant layer is shown to be critical for optimizing doping and the efficiency of the device.     

Fabrication of silicon nanowire devices for ultrasensitive, label-free, real-time detection of biological and chemical species

Detection and quantification of biological and chemical species are central to many areas of healthcare and the life sciences, ranging from diagnosing disease to discovery and screening of new drug molecules. Semiconductor nanowires configured as electronic devices have emerged as a general platform for ultra-sensitive direct electrical detection of biological and chemical species. Here we describe a detailed protocol for realizing nanowire electronic sensors. First, the growth of uniform, single crystal silicon nanowires, and subsequent isolation of the nanowires as stable suspensions are outlined. Second, fabrication of addressable nanowire device arrays is described. Third, covalent modification of the nanowire device surfaces with receptors is described. Fourth, an example modification and measurements of the electrical response from devices are detailed. The silicon nanowire (SiNW) devices have demonstrated applications for label-free, ultrasensitive and highly-selective real-time detection of a wide range of biological and chemical species, including proteins, nucleic acids, small molecules and viruses.     

Highly sensitive measurements of PNA-DNA hybridization using oxide-etched silicon nanowire biosensors

The highly sensitive and sequence-specific detection of single-stranded oligonucleotides using nonoxidized silicon nanowires (SiNWs) is demonstrated. To maximize device sensitivity, the surface of the SiNWs was functionalized with a densely packed organic monolayer via hydrosilylation, subsequently immobilized with peptide nucleic acid (PNA) capable of recognizing the label-free complementary target DNA. Because of the selective functionalization of the SiNWs, binding competition between the nanowire and the underlying oxide is avoided. Transmission electron microscopy was conducted to clearly differentiate the SiNW surface before and after removal of SiO₂. Fluorescence microscopy was used to further realize the selectivity of the oxide-etched chemistry on the SiNWs and sequence specificity of PNA-DNA hybridization. The concentration-dependent resistance change measurements upon hybridization of PNA-DNA show that detection limit down to 10 fM can be obtained. The SiNW devices also reveal the capability of an obvious discrimination against mismatched sequences. Among several efforts being made to improve detection sensitivity, this work addresses one significant issue regarding surface functionalization which enables highly sensitive biomolecular sensing with SiNWs.     

A highly sensitive, label-free gene sensor based on a single conducting polymer nanowire

A prerequisite for exploiting sensing devices based on semiconductor nanowires is ultra-sensitive and selective direct electrical detection of biological and chemical species. Here, we constructed a transducer based on copolymer of poly(3,4,-ethylenedioxythiophene) (PEDOT) and carboxylic group functionalised PEDOT single nanowire in between gold electrodes, followed by covalent attachment of amino-modified probe oligonucleotide. The target ODNs specific to Homo sapiens Breast and ovarian cancer cells were detected at femtomolar concentration and incorporation of negative controls (non-complementary ODN) were clearly discriminated by the sensor. The ex situ measurements were performed by using two terminal device setup and the changes in the interface of the nanowire associated with the association or dissociation of ODNs were measured as change in resistance. In addition, in situ measurements were performed by utilizing scanning ion conductance microscopy to measure the change in resistance of probe modified nanowire upon addition of different concentration of target ODNs in presence of relevant buffer. The constructed, nano sensor showed highly sensitive concentration dependent resistance change.     

Encoded metal nanoparticle-based molecular beacons for multiplexed detection of DNA

In this paper we describe a molecular beacon format assay in which encoded nanowire particles are used to achieve multiplexing. We demonstrate this principle with the detection of five viral pathogens; Hepatitis A virus, Hepatitis C virus, West Nile Virus, Human Immune Deficiency virus and Severe Acute Respiratory Syndrome virus. Oligonucleotides are designed complementary to a target sequence of interest containing a 3′ universal fluorescence dye. A 5′ thiol causes the oligonucleotides to self-assemble onto the metal nanowire. The single-stranded oligonucleotide contains a self-complementary hairpin stem sequence of 10 bases that forces the 3′ fluorophore to come into contact with the metallic nanowire surface, thereby quenching the fluorescence. Upon addition of target DNA, there is hybridization with the complementary oligonucleotides. The resulting DNA hybrid is rigid, unfolds the hairpin structure, and causes the fluorophore to be moved away from the surface such that it is no longer quenched. By using differently encoded nanowires, each conjugated with a different oligonucleotide sequence, multiplexed DNA assays are possible using a single fluorophore, from a multiplexed RT-PCR reaction.     

Less is More: Dopant‐Free Hole Transporting Materials for High‐Efficiency Perovskite Solar Cells

Perovskite solar cells have delivered power conversion efficiency beyond 22% in less than seven years, implying the potential for the paradigm shift of low‐cost photovoltaics with high efficiency and low embedded energy. Besides the “perovskite fever,” the development of new hole transport materials (HTM), especially dopant‐free HTMs, is another research hotspot. This is because the currently used HTMs, such as spiro‐OMeTAD derivatives, require additional chemical doping process to ensure sufficient conductivity and proper ionic potential level for efficient hole transport and collection. However, the commonly used dopants are volatile and hygroscopic which not only increase the complexity and cost of device fabrication but also deteriorate the device stability. So far, there have been several reviews on new HTMs, but review or analysis on dopant‐free HTMs is scarce. In this review, all reported dopant‐free HTMs are categorized into four primary different types and lessons will be learned during the separate discussions. The stability test behavior of all the intrinsic HTMs will be evaluated directly. In the end, the correlations between the properties of the intrinsic HTMs and parameters of the devices will be plotted to shed light on the future direction of development of this field.     

Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support^1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks⁴. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons⁵ in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires⁶ randomly distributed on a glass substrate⁷. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization^8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity^5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate^9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy^9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.     

Band-gap engineering of halogenated silicon nanowires through molecular doping

In this work, we address the effects of molecular doping on the electronic properties of fluorinated and chlorinated silicon nanowires (SiNWs), in comparison with those corresponding to hydrogen-passivated SiNWs. Adsorption of n-type dopant molecules on hydrogenated and halogenated SiNWs and their chemisorption energies, formation energies, and electronic band gap are studied by using density functional theory calculations. The results show that there are considerable charge transfers and strong covalent interactions between the dopant molecules and the SiNWs. Moreover, the results show that the energy band gap of SiNWs changes due to chemical surface doping and it can be further tuned by surface passivation. We conclude that a molecular based ex-situ doping, where molecules are adsorbed on the surface of the SiNW, can be an alternative path to conventional doping.

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Graphical abstract Molecular doping of halogenated silicon nanowires

     

The label free DNA sensor using a silicon nanowire array

Biosensors based on silicon nanowire (Si-NW) promise highly sensitive dynamic label free electrical detection of various biological molecules. Here we report Si-NW array electronic devices that function as sensitive and selective detectors of as synthesized 2D DNA lattices with biotins. The Si-NW array was fabricated using top-down approach consists of 250 nanowires of 20 μm in length, equally spaced with an interval of 3.2 μm. Measurements of photoresistivity of the Si-NW array device with streptavidin (SA) attached on biotinylated DNA lattices at different concentration were observed and analyzed.. The conductivity in the DNA lattices with protein SA shows significant change in the photoresistivity of Si-NW array device. This Si-NW based DNA sensor would be one of very efficient devices for direct, label free DNA detection and could provide a pathway to immunological assays, DNA forensics and toxin detection in modern biotechnology.     

Highly Interconnected Si Nanowires for Improved Stability Li‐Ion Battery Anodes

Silicon exhibits the largest known capacity for Li insertion in anodes of Li‐ion batteries. However, because of large volume expansion/phase changes upon alloying, Si becomes powder‐like after a few charge‐discharge cycles. Various approaches have been explored in the past to circumvent this problem, including the use of nanomaterials, particularly Si nanowires. However, even though nanowires resist cracking very well, anodes based on Si nanowires still see their original capacity fade away upon cycling, because of wire detachment from the substrate, due to the stress generated at their roots upon alloying with Li. Here, we present a silicon nanowire growth strategy yielding highly interconnected specimens, which prevents them from being individually detached from the substrate. We report a ～100% charge retention after 40 cycles at C/2 rate, without charging voltage limitation. We also show that our anodes can be cycled at 8C rates without damage and we grow nanowires with a density of 1.2 mg/cm², yielding anodes delivering a 4.2 mAh/cm² charge density. Finally, we point out that a better understanding of the interactions of silicon with electrolytes is needed if the field is to progress in the future.     

Breakthrough Carbon Nanotube–Silicon Heterojunction Solar Cells

The latest advances in carbon nanotube–silicon heterojunction solar cells are combined with a new doping protocol based on the outstanding electron withdrawing properties and excellent silicon surface passivation ability of sulfonated polytetrafluoroethylene (Nafion). Using this new dopant for carbon nanotube–silicon solar cells, advanced substrate design, and an optimized antireflective texture fast etch with organic base, breakthrough performance is obtained from research grade devices with active areas of 1 and 5 cm², which yield power conversion efficiencies of 17.2 and 15.5%, respectively.     

Morphology of living cells cultured on nanowire arrays with varying nanowire densities and diameters

Vertical nanowire arrays are increasingly investigated for their applications in steering cell behavior. The geometry of the array is an important parameter, which influences the morphology and adhesion of cells. Here, we investigate the effects of array geometry on the morphology of MCF7 cancer cells and MCF10A normal-like epithelial cells. Different gallium phosphide nanowire array-geometries were produced by varying the nanowire density and diameter. Our results show that the cell size is smaller on nanowires compared to flat gallium phosphide. The cell area decreases with increasing the nanowire density on the substrate. We observed an effect of the nanowire diameter on MCF10A cells, with a decreased cell area on 40 nm diameter nanowires, compared to 60 and 80 nm diameter nanowires in high-density arrays. The focal adhesion morphology depends on the extent to which cells are contacting the substrate. For low nanowire densities and diameters, cells are lying on the substrate and we observed large focal adhesions at the cell edges. In contrast, for high nanowire densities and diameters, cells are lying on top of the nanowires and we observed point-like focal adhesions distributed over the whole cell. Our results constitute a step towards the ability to fine-tune cell behavior on nanowire arrays.     

Cell guidance by magnetic nanowires

The phenomenon of contact guidance on thin fibers has been known since the beginning of the 20th century when Harrison studied cells growing on fibers from spider's web. Since then many studies have been performed on structured surfaces and fibers. Here we present a new way to induce guidance of cells or cell processes using magnetic nanowires. We have manufactured magnetic Ni-nanowires (200 nm in diameter and 40 μm long) with a template-based electro-deposition method. Drops of a nanowire/ethanol suspension were placed on glass cover slips. The nanowires were aligned in an external magnetic field and adhered to the cover slips after evaporation of the ethanol. When the wires had adhered, the magnetic field was removed. L929 fibroblasts and dissociated dorsal root ganglia (DRG) neurons from mice were cultured on the nanowire-coated cover slips for 24 h and 72 h respectively. The fibroblasts were affected by the aligned nanowires and displayed contact guidance. Regenerated axons also displayed contact guidance on the wires. There were no overt signs of toxicity caused by Ni-wires. Aligned magnetic nanowires can be useful for lab-on-a-chip devices and medical nerve grafts.     

Folic acid incorporated nitrogen-doped carbon dots as a turn-on fluorescence probe for homocysteine detection

This study describes the development of a low-cost fluorescence assay for detecting homocysteine (Hcy) without the interference of cysteine and glutathione using carbon quantum dots. Herein nitrogen-doped carbon quantum dots (NCDs) were synthesized from citric acid as the carbon source and urea as the dopant using a one-pot microwave-assisted method. The obtained NCDs were incorporated with folic acid (FA) by the direct ex situ addition method and were used as a fluorescence probe to detect Hcy. The probe exhibited a fluorescence turn-on response with increased Hcy concentration up to 50 μM with a limit of detection of 2.276 μM. The point of care detection of Hcy using the probe was also tested with a paper-based assay strip.     

一株促甲烷氧化假单胞菌Pseudomonas putida P7的分离及电活性特征

微生物胞外呼吸是厌氧环境中控制性能量代谢方式,直接驱动着C、N、S、Fe等关键元素的生物地球化学循环。微生物纳米导线（Microbial nanowires）的发现,被认为是微生物胞外呼吸的里程碑事件,推动了电微生物学（Electromicrobiology）的形成与发展。微生物纳米导线是一类由微生物合成的,具有导电性的纤维状表面附属结构。通过细菌纳米导线,微生物胞内代谢产生的电子可以长距离输送到胞外受体或其他微生物,改变了电子传递链仅仅局限于细胞胞内的认识,从而大大拓展了微生物-胞外环境互作的范围。微生物纳米导线的良好导电性,赋予了其作为天然纳米材料的广阔应用前景。目前,微生物纳米导线的导电机制、生态功能及其在生物材料、生物能源、生物修复及人体健康多领域的应用,已经成为新兴电微生物学的前沿与热点。然而,微生物纳米导线的生物学、生态学功能尚不清楚,它的电子传递机制仍存在分歧。本文在系统性总结微生物纳米导线性质、功能的基础上,以Geobacter sulfurreducens和Shewanella oneidensis纳米导线为模型,详细阐述了纳米导线的组成与结构、表征与测量方法、导电理论（类金属导电学说与电子跃迁学说）及其潜在的应用,最后提出了未来微生物纳米导线研究的重点方向、挑战与机遇。