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Friday 12 May 2017, 14:30, Prof.dr. G. Berkhoff-zaal

PhD Defence Janneke Veerbeek

Janneke Veerbeek is a PhD student in the MESA+ research group Molecular Nanofabrication. Her supervisor is Jurriaan Huskens.

MONOLAYER FUNCTIONALIZATION OF SILICON MICRO AND NANOWIRES: TOWARDS SOLAR-TO-FUEL AND SENDING DEVICES

Silicon is an attractive semiconductor material for wide­ranging applications, especially when taking advantage of the larger surface area of silicon micro and nanowires. Surface functionalization with self­assembled monolayers of (in)organic molecules can be employed to tune the functionality of a substrate towards a desired application. Specifically, solar­to­fuel and sensing devices highly benefit from the use of oxide­free monolayers, since any silicon oxide layer functions as an insulating layer and retards electrical contact with the substrate. For this purpose, hydrosilylation can be used to couple terminal unsaturated carbon­carbon bonds, i.e., 1­alkenes or 1­alkynes, onto H­terminated Si which leads to direct Si­C bond formation. The research described in this thesis aims at the formation of molecular monolayers on H­terminated silicon micro and nanowires for solar­to­fuel and sensing devices.

In the first part of this thesis (Chapters 3­4), oxide­free monolayers have been applied onto silicon micro and nanowires for solar­to­fuel devices. Chapter 3 has reported the use of molecular monolayers for simultaneous passivation and catalyst coupling. A 1­tetradecyne monolayer functioned as an electrical passivation layer, which increased the efficiency of micropillar­based solar cells. At the same time, secondary functionalization was demonstrated with a model catalyst by copper­catalyzed click chemistry onto a 1,8­nonadiyne monolayer, which could be used in the future for (photo)catalyst coupling for efficient solar fuel production. In Chapter 4, nanowires have been fabricated by metal­assisted chemical etching (MACE) to benefit from an even larger surface area compared to microwires. After fabrication, highly doped nanowires were created by three different monolayer doping techniques, where the total doping dose inside the nanowires could be tuned by changing the porosity of the nanowires.

The second part of this thesis (Chapters 5­6) has described the selective functionalization of silicon nanowires. Chapter 5 has discussed the spatioselective functionalization of silicon nanowires without the use of a masking material, based on alternating steps of MACE and monolayer formation. This resulted in nanowires with a 1,8­nonadiyne monolayer on the upper segments only, as seen by a contrast difference in scanning electron microscopy images. Secondary functionalization with azide­functionalized nanoparticles confirmed this observation by a threefold higher particle density on the top segments compared to the lower segments without a monolayer. In Chapter 6, material­selective functionalization of silicon nanowire sensors was targeted to prevent loss of analyte and increase the sensor’s sensitivity. However, hydrosilylation resulted in nonselective functionalization, which required the use of an extra wet etching step to remove the monolayer from the oxidized areas. This increased the selectivity towards functionalization of the silicon nanowires, which were then functionalized with probe PNA for tumor DNA detection.

In the third part of this thesis (Chapters 7­8), electrochemistry on oxide­free monolayers has been tested. Chapter 7 has described the electron transfer of redox­active guest molecules at silicon electrodes with β­cyclodextrin host molecules. Whereas the packing density and extent of oxide formation were comparable for monolayers on lowly doped p­type and highly doped p++ substrates, the electron transfer was more favorable on p++ substrates. This indicated that the electrochemical response on the host layer is not only determined by the composition of the monolayer but also by the doping level of the substrate. This proof of principle was further explored in Chapter 8, in which hydrogen evolution catalysts were immobilized on silicon and gold electrodes. Hydrosilylation was used to couple an alkyne­functionalized catalyst covalently onto H­terminated silicon. Alternatively, β­cyclodextrin host monolayers were used to immobilize a guest-functionalized catalyst on gold and silicon electrodes by supramolecular interactions. Whereas catalyst immobilization was confirmed for both routes, preliminary results only hinted at reduction events for both catalysts, and more research is needed to verify their catalytic activity after immobilization.

In summary, the results described in this thesis demonstrate the versatility of molecular monolayers on H­terminated silicon structures. When applying a monolayer by hydrosilylation, the absence of an insulating oxide layer allows electrical contact between the functionalized headgroup of the monolayer and the substrate. This enables the fabrication of solar­to­fuel devices, in which molecular monolayers ensure electrical passivation of the surface, a controllable doping concentration, covalent or noncovalent immobilization of catalysts, and spatioselective functionalization to couple different catalysts onto the same device. For sensing devices, oxide­free monolayers establish a higher stability of the sensor owing to the direct Si­C bonds, a higher sensitivity due to the selective functionalization of the sensing area only, and a higher specificity when using supramolecular chemistry to make an analyte­specific sensor. These findings offer new perspectives for the development of stabilized silicon micro/nanosystems with engineered functionalities.