CHAPTER 14

Perspective and Challenge

FRANKLIN (FENG) TAO AND STEVEN L. BERNASEK

In the past two decades, research in the area of functionalization of semiconductor surfaces has elucidated reaction mechanisms of organic molecules on semiconductor surfaces in many examples. These studies have also led to designed semiconductor surfaces tethered with functional organic materials and biospecies, and have demonstrated molecule-based semiconductor devices and biosensing techniques. However, there remain many open questions.

In order to grow multilayer, three-dimensional, organic architectures, or to immobilize biospecies on semiconductor surfaces, a functionalized surface with outward facing functional groups is necessary. A chemically homogeneous organic monolayer is crucial for significant signal response. To form such a surface with a single functional group facing outward, a bifunctional molecule is typically used. In fact, obtaining methods that exhibit a high selectivity for attachment of only one functionality of the bifunctional molecule to the semiconductor surface is crucial. A protection group to passivate one of the two functional groups can be used in the direct reaction with the semiconductor surfaces. This would be followed by activation of the protected group in the attachment of the second layer of organic molecules. This approach can enhance surface selectivity for some bifunctional molecules. However, the selection of a specific protection group is challenging, and this extra step can often result in damage to the desired functionality. Thus, actually this strategy largely limits the growth of semiconductor surface with pure upward functional group for further applications. More efforts in developing new strategies are necessary.

Many previous studies, including mechanistic studies of organic molecules and the immobilization of organic materials and biospecies, have been performed at room temperature or low temperature. In practice, operational conditions of molecular electronics devices and biosensors at temperatures higher than room temperature are unavoidable. Thus, studies of the thermal evolution of these systems are apiece of information that is important but absent so far for many real applications. In addition, how ambient environments (air, humidity) impact chemical stability of the functionalized surfaces is another important issue to be addressed. Many of the modification bonds (Si–C, Si–O, or Si–N linkages) are actually sensitive to reactive gases such as water vapor and O₂. How such a “slow” but “accumulating” chemical reaction degrades the function of molecule-based devices and sensors should be addressed for real applications in the near future. In situ studies of these functionalized semiconductor surfaces under reaction conditions were almost impossible in the past decades. However, the recent advance in in situ techniques capable of working under ambient conditions or reactive conditions has made such studies possible. For example, ambient pressure XPS allows the possibility to measure surface composition and study oxidation state and identify chemical environment of surfaces under a pressure of gaseous reactant of 10 Torr or under a liquid layer with a thickness of a few nanometers.

In addition, most of the previous studies were performed on flat homogeneous surfaces. With the advance of nanolithography, functionalization of semiconductor surfaces at micro- or even nanoscale can be realized by using nanopatterned substrates. Through selective passivation of a semiconductor surface, organic molecules can be selectively attached to unpassivated surfaces to form organic or bio-nanopatterns. Such nanostructures could provide spatial selectivity in biosensing technology. This could be an exciting area in the development of biosensing techniques and biomimetic devices.

Another exciting future area in the functionalization of semiconductor surfaces is the incorporation of synthesis of nanoparticles through colloidal chemistry into functions of semiconductor surfaces. Metal nanoparticles are typically coated with a layer of surfactant such as cationic acetyl trimethyl ammonium bromide (C-TAB). The outward functional group of surfactant molecules on metal nanoparticles could be used as a linkage to covalently bind to the outward functional groups tethered on semiconductor surfaces, modifying the semiconductor surface and bringing size-dependent physical, optoelectronic, and chemical properties to the functionalized semiconductor surfaces. This also provides another platform for functionalization of the semiconductor surface since many unique chemical and physical properties of metal or semiconductor nanoparticles could be introduced to the semiconductor surface system.

In any event, there are a number of areas for further research in this exciting field of materials chemistry. This book has provided a survey and review of much of the work that has been carried out in the field of semiconductor surface modification. Applications in organic and eventually molecular electronics, surface modification of the biological and mechanical properties of the semiconductor surface, and the development of chemical and biological sensor devices are apparent in the work described here. There remains much to be done, however, as we have indicated only briefly in these concluding statements. The field of semiconductor surface functionalization and modification will continue to hold excitement for researchers for many years to come.