Table 2 Brief comparison of different SiNWs alignment methods
From: Recent Advances in Silicon Nanowire Biosensors: Synthesis Methods, Properties, and Applications
Alignment type | Alignment method | Inter-NW distance, alignment yield and control of NW density | Merits | Demerits | References |
|---|---|---|---|---|---|
Langmuir–Blodgett alignment | Parallel alignment of SiNWs during uniaxial compression of Langmuir–Blodgett trough | 8–10 NW/mm; alignment yield is about 80–90 %. SiNW density is controlled by the compression of Langmuir–Blodgett trough | Alignment can be useful has a substrates spanning several cm2 in area. Cross-sINW structure is attainable using sequential rounds of Langmuir–Blodgett alignment. | Irreproducibility in the alignment direction of sINWs can lead to bad/weak end-to-end registration with the source and drain electrodes. It is only effective with SiNWs with diameter >15 nm. Almost impossible to control and coordinate the number of SiNWs bridging the source and drain contact electrodes | [17] |
Blown–bubble alignment | Suspension of SiNW–polymer solution blown into a bubble using gas flow | ca. 1 NW/3 mm Alignment yield: 90 %. SiNW density is organized by varying the concentration of SiNWs in the SiNW–polymer suspension solution. | Alignment method can be applied to various SiNW materials like planar, plastic, curved. Alignment feasible up to various length scales (from mm to m). | Needs surface functionalization of SiNWs with epoxy group to form SiNW-polymer film, which may reduce the availability and efficiency of SiNW surface in terms of immobilization of biorecognition element Hard to control the number of SiNWs bridging the source and drain contact electrodes | [18] |
Flow-based alignment | Microfluidic flow-driven shear forces, where the adsorption of NWs is facilitated by surface charge. | 2–3 NWs/mm Alignment yield: 80 %. SiNW density is controlled by flow duration. | Cross-SINW arrays and equilateral triangles can be constructed using a chemically patterned surface and sequential layer-by-layer assembly steps with different flow directions. Alignment needs small sample volume of SiNWs (mL). | Alignment is restricted to planar substrates and to small length scales ranging from few mm to cm. It is only applicable to SiNWs with diameter >15 nm. It is so difficult to control the number of SiNWs bridging the source and drain contact electrodes. | [19] |
Electric-field based alignment | It involves balance of hydrodynamic and dielectrophoretic forces. | 1 NW/12 mm Alignment yield: >98 %. NW density is controlled by the number of patterned electrode sites in a specific area. | There are no available incorporation issues of SiNWs with the source and drain contact electrodes. Surface modification of SiNWs can be done before alignment. Each SiNW can be worked on singly from an electrical contact standpoint. | It demands precise control of the hydrodynamic and dielectrophoretic forces. Dissimilarities in the physiochemical properties of SiNWs can truncate the alignment process. Alignment only possible for small area (from mm2 to cm2). The quality and density of the SiNW produced is low as compared to other methods. | [20] |
Contact printing alignment | Shear stress during the sliding of donor (the growth substrate) and receiver substrates. An intermediate step such as stamp transfer using a roller can also be employed (roll-transfer printing). | 4–8 NW/mm Alignment yield: 80–90 % NW density can be controlled by changing the receiver substrate with various functional groups. | Alignment viable with several SiNW materials and can be applied to diverse substrates (silicon, plastic and rubber etc.). Also applicable to SiNWs with diameter <15 nm Multilayer functional device structures are achievable by iterative contact printing and device fabrication steps. Roll-transfer printing method can be operated in a continuous fashion. Strained PDMS stamp can be applied to improve the efficiency of alignment yield and SiNW density. | Lack of control in breakage of SiNWs during the transfer process, resulting in distribution of NW lengths. The length of SiNWs printed on the receiver substrate is characteristically less than the length of SiNWs on the growth substrate. It is difficult to control the number of SiNWs bridging the source and drain contact electrodes. |