Semiconducting Silicon Nanowires for Biomedical Applications
Introduction
Silicon remains the unquestionable mainstay of the electronic device industry, with a constant scrutiny of its use under the lens of Moore’s Law and an ongoing reduction in feature size and corresponding device dimensions (Mack, 2011). At the same time, knowledge of its fundamental properties have also benefi ted from the expansive growth in nanoscience and nanotechnology, with a broad spectrum of investigations being reported for this elemental semiconductor in one- dimensional nanowire form. Early experiments probed its ability to act as a chemical sensor (Cui et al. , 2001) and high- density p- n junction and transistor array (Cui and Lieber, 2001; Huang et al. , 2001). However, while bulk crystalline Si is traditionally viewed as bio- inert, the unique geometry of Si nanowires (SiNW), their diverse surface chemistry, as well as associated process engineering, have provided a boon of sorts in terms of fundamental studies of relevance to its ultimate application in the fi eld of biomedical devices. In this vein, SiNW have demonstrated some amazing properties in terms of biological functions to which it can contribute and analyze. These range from the transfection of individual cells (Kim et al. , 2007) to the detection of electrical signals within a cardiac cell (Tian et al. , 2010). Thus, it is the biomedical relevance of semiconducting SiNW that is the focus of this book, with a diverse range of experts from a number of institutions across the globe assembled to tackle the key themes of this area of research.
Origins of silicon nanowires
An overview of these issues will be highlighted momentarily, but let us begin with a brief historical perspective. The genesis of SiNW can be viewed as originating with seminal efforts regarding relatively larger diameter cylindrical structures of Si in the micrometer dimension, perhaps better known as Si whisker technology (Levitt, 1971). A key component of this effort was the fundamental discovery of the Vapor-Liquid-Solid (VLS) method, whereby the dimensions of a catalyst particle (such as Au) take advantage of the limited solubility of Si in a metal silicide liquid phase (e.g. Si(s) precipitating in AuSi
(l)) at the proper temperature and reactant concentrations to form a crystalline Si microwire (Wagner and Ellis, 1964). It was Lieber and co-workers who had the prescient realization that proper reduction of the catalyst dimension from the macro- to the nanoscale could yield construction of the target well-defi ned cylindrical nanowire constructs (Morales et al. , 1998), thereby opening the door for the expansive number of papers on this nanomaterial that have subsequently appeared.
In that regard, let us look at the evolution of interest in the topic of silicon nanowires within the last 15 years. This is best exemplifi ed by a search of the phrase ‘silicon nanowire’ appearing in the citations in the Web of Science database (Thomson Reuters) on an annual basis (up to, but not including, 2012) (see Fig. 1.1 ).
The number of citations containing ‘silicon nanowire’ in terms of content has clearly increased in an exponential manner, to a total of more than 7000 since 1997. From a biomedical context, inclusion of the term ‘cell’ along with ‘silicon nanowire’ results in a similar explosive growth curve, starting from a mere two references in 1999 to a value of 213 alone in 2011.
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Introduction
Silicon remains the unquestionable mainstay of the electronic device industry, with a constant scrutiny of its use under the lens of Moore’s Law and an ongoing reduction in feature size and corresponding device dimensions (Mack, 2011). At the same time, knowledge of its fundamental properties have also benefi ted from the expansive growth in nanoscience and nanotechnology, with a broad spectrum of investigations being reported for this elemental semiconductor in one- dimensional nanowire form. Early experiments probed its ability to act as a chemical sensor (Cui et al. , 2001) and high- density p- n junction and transistor array (Cui and Lieber, 2001; Huang et al. , 2001). However, while bulk crystalline Si is traditionally viewed as bio- inert, the unique geometry of Si nanowires (SiNW), their diverse surface chemistry, as well as associated process engineering, have provided a boon of sorts in terms of fundamental studies of relevance to its ultimate application in the fi eld of biomedical devices. In this vein, SiNW have demonstrated some amazing properties in terms of biological functions to which it can contribute and analyze. These range from the transfection of individual cells (Kim et al. , 2007) to the detection of electrical signals within a cardiac cell (Tian et al. , 2010). Thus, it is the biomedical relevance of semiconducting SiNW that is the focus of this book, with a diverse range of experts from a number of institutions across the globe assembled to tackle the key themes of this area of research.
Origins of silicon nanowires
An overview of these issues will be highlighted momentarily, but let us begin with a brief historical perspective. The genesis of SiNW can be viewed as originating with seminal efforts regarding relatively larger diameter cylindrical structures of Si in the micrometer dimension, perhaps better known as Si whisker technology (Levitt, 1971). A key component of this effort was the fundamental discovery of the Vapor-Liquid-Solid (VLS) method, whereby the dimensions of a catalyst particle (such as Au) take advantage of the limited solubility of Si in a metal silicide liquid phase (e.g. Si(s) precipitating in AuSi
(l)) at the proper temperature and reactant concentrations to form a crystalline Si microwire (Wagner and Ellis, 1964). It was Lieber and co-workers who had the prescient realization that proper reduction of the catalyst dimension from the macro- to the nanoscale could yield construction of the target well-defi ned cylindrical nanowire constructs (Morales et al. , 1998), thereby opening the door for the expansive number of papers on this nanomaterial that have subsequently appeared.
In that regard, let us look at the evolution of interest in the topic of silicon nanowires within the last 15 years. This is best exemplifi ed by a search of the phrase ‘silicon nanowire’ appearing in the citations in the Web of Science database (Thomson Reuters) on an annual basis (up to, but not including, 2012) (see Fig. 1.1 ).
The number of citations containing ‘silicon nanowire’ in terms of content has clearly increased in an exponential manner, to a total of more than 7000 since 1997. From a biomedical context, inclusion of the term ‘cell’ along with ‘silicon nanowire’ results in a similar explosive growth curve, starting from a mere two references in 1999 to a value of 213 alone in 2011.
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