not yet been fully elucidated, allowing only approximate comparisons of quenching
efficiencies to be made. The effects of surface chemistry upon the physical morphology of the nanocrystalline architecture, the breadth and depth of porosity, the
magnitude of surface area, and the electronic character of the porous silicon also
remain to be more completely investigated. By establishing how various functionalization reactions and surface terminations affect the material properties, more
precise control of the surface states can be achieved with minimal damage or
transformation of the silicon nanocrystallites. Research into the mechanisms of
these reactions may yield new insight into the general nature of semiconductor interfaces and reveal additional strategies for the functionalization of these
surfaces.
Progress in the synthesis of stable, functional materials has generated, within
the past decade, a number of wet chemical methods for the modification of porous
silicon surfaces [22, 35]. In particular, the formation of organic monolayers bound
via SiaC bonds has garnered more attention in recent years owing to the low polarity and kinetically inert nature of these largely covalent linkages [29, 36–38].
Advances in the chemistry of hydride-terminated nanocrystalline silicon for the
preparation of such monolayers have resulted in a plethora of reactions, including
thermal [20, 39, 40], chemical [21, 41–47], electrochemical [48–51], and photochemical [52, 53] methods, that are capable of producing selectively functionalized
materials upon demand (Figure 16.5). Some of these methods are adapted from
similar reactions known for molecular silicon compounds [54], illuminating some
parallels between the chemistries of solution-phase hydridosilanes and solid-state,
hydride-terminated surfaces. Others are developed from procedures known to
functionalize flat, single-crystal silicon surfaces [55–62], illustrating reactions that
may proceed on all varieties of hydride surface, regardless of morphology or orientation. Several of these reactions have no known molecular or flat-surface analogs, instead employing reactive properties unique to nanocrystalline silicon to
drive the formation of surface bonds. While earlier strategies for the derivatization
of porous silicon focused upon the reactions of hydride and oxide surfaces to form
SiaO bonds [22], interest in this approach has generally waned owing to the
generally inferior stability of these monolayers and the relatively narrow range of
functionalities that can be imparted to the silicon surfaces.
Preparation and analysis of porous silicon substrates functionalized with organic
monolayers can be readily accomplished by a variety of procedures using equipment and techniques familiar to most chemists. The native material can be handled in air for brief periods without significant oxidation and contamination, but
porous silicon keeps much longer when maintained under inert atmosphere. Most
manipulations and chemical treatments can be completed using straightforward
Schlenk and glovebox practices. The high surface area of porous silicon permits
characterization by reliable, convenient analytical tools such as transmission FTIR,
diffuse reflectance infrared Fourier transform (DRIFT), solid-state NMR, and X-ray
photoelectron spectroscopies. In fact, owing to its relative ease of characterization,
porous silicon may serve as a ‘test bed’ for chemical reactions to be attempted upon
flat silicon substrates. A host of functional groups have been successfully incorpo16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
524
efficiencies to be made. The effects of surface chemistry upon the physical morphology of the nanocrystalline architecture, the breadth and depth of porosity, the
magnitude of surface area, and the electronic character of the porous silicon also
remain to be more completely investigated. By establishing how various functionalization reactions and surface terminations affect the material properties, more
precise control of the surface states can be achieved with minimal damage or
transformation of the silicon nanocrystallites. Research into the mechanisms of
these reactions may yield new insight into the general nature of semiconductor interfaces and reveal additional strategies for the functionalization of these
surfaces.
Progress in the synthesis of stable, functional materials has generated, within
the past decade, a number of wet chemical methods for the modification of porous
silicon surfaces [22, 35]. In particular, the formation of organic monolayers bound
via SiaC bonds has garnered more attention in recent years owing to the low polarity and kinetically inert nature of these largely covalent linkages [29, 36–38].
Advances in the chemistry of hydride-terminated nanocrystalline silicon for the
preparation of such monolayers have resulted in a plethora of reactions, including
thermal [20, 39, 40], chemical [21, 41–47], electrochemical [48–51], and photochemical [52, 53] methods, that are capable of producing selectively functionalized
materials upon demand (Figure 16.5). Some of these methods are adapted from
similar reactions known for molecular silicon compounds [54], illuminating some
parallels between the chemistries of solution-phase hydridosilanes and solid-state,
hydride-terminated surfaces. Others are developed from procedures known to
functionalize flat, single-crystal silicon surfaces [55–62], illustrating reactions that
may proceed on all varieties of hydride surface, regardless of morphology or orientation. Several of these reactions have no known molecular or flat-surface analogs, instead employing reactive properties unique to nanocrystalline silicon to
drive the formation of surface bonds. While earlier strategies for the derivatization
of porous silicon focused upon the reactions of hydride and oxide surfaces to form
SiaO bonds [22], interest in this approach has generally waned owing to the
generally inferior stability of these monolayers and the relatively narrow range of
functionalities that can be imparted to the silicon surfaces.
Preparation and analysis of porous silicon substrates functionalized with organic
monolayers can be readily accomplished by a variety of procedures using equipment and techniques familiar to most chemists. The native material can be handled in air for brief periods without significant oxidation and contamination, but
porous silicon keeps much longer when maintained under inert atmosphere. Most
manipulations and chemical treatments can be completed using straightforward
Schlenk and glovebox practices. The high surface area of porous silicon permits
characterization by reliable, convenient analytical tools such as transmission FTIR,
diffuse reflectance infrared Fourier transform (DRIFT), solid-state NMR, and X-ray
photoelectron spectroscopies. In fact, owing to its relative ease of characterization,
porous silicon may serve as a ‘test bed’ for chemical reactions to be attempted upon
flat silicon substrates. A host of functional groups have been successfully incorpo16 Recent Developments in the Chemistry and Chemical Applications of Porous Silicon
524