Daniel P. Puzzo

Nanofabrication by self-assembly

The self-assembly paradigm in chemistry, physics and biology has matured scientifically over the past two-decades to a point of sophistication that one can begin to exploit its numerous attributes in nanofabrication. In what follows we will take a brief look at current thinking about self-assembly and with some recent examples taken from our own work examine how nanofabrication has benefited from self-assembly.

Towards the Photonic Nose: A Novel Platform for Molecule and Bacteria Identification

Adv. Mater. 2010, 22, 1351–1354 2010 WILEY-VCH Verlag G O N The olfactory system has been recognized for its ability to identify airborne molecules using a combinatorial response, in which a library of activated olfactory receptor neurons gives a unique fingerprint for each type of odorant. Mimics of the olfactory system, dubbed artificial noses that enable identification of vapor phase compounds are currently of great technological interest for odor analysis in areas that include flavor and fragrance, food and beverage, packaging, pharmaceutical, cosmetic and perfume, narcotic and disease diagnostics. While artificial noses for the detection of gas-phase molecules based on modulation of electrical and gravimetric properties are well documented, by contrast, optical noses based on modulation of optical properties are scarcer, even with a well-developed field of optical sensing materials available. Herein, we present the concept of the photonic nose, a novel combinatorial sensing platform whose operating principle is based on molecule-induced modulation of the optical Bragg diffraction properties of a pixelated nanoparticle 1D photonic crystal, in which each pixel has different surface-energy properties enabled through selective chemical functionalization. The photonic nose is a straightforward and low-cost, environmentally friendly and defect-tolerant combinatorial colorimetric sensor that is able to detect and discriminate vapor species like small molecules and bacteria volatiles with a simple digital-camera color-imaging system. These are considered the first steps towards the use of a photonic nose in chemical sensing and disease diagnostics. The development of an artificial nose based on the modulation of optical signals was first reported by Dickinson et al., using a fluorescence-based approach with a fiber-optics array. The use of self-encoded bead-assisted detection was a major breakthrough, opening the way to a number of applications such as attomolar DNA detection. Rakow and Suslick were the first to report an artificial nose based on a colorimetric approach by using an array of metal porphyrins, in which each type of porphyrin shows a different coordination constant with the vapor analytes, leading to unique color-change patterns upon binding of vapor-phase ligands or solvatochromic-induced effects. Further development of the concept with the incorporation of a larger variety of sensing species allowed discrimination of 100 volatile organic compounds. The approach described herein is distinct to the above and is instead based on the use of functional 1D photonic crystals comprised of multilayers of alternating refractive index, also known as Bragg stacks (BS). Functionality can be introduced into BS by the incorporation of (meso)porosity into the layered structure. This approach provides photoniccrystal architectures with a high surface area and tunable color upon infiltration and capillary condensation of solvent vapors. The color changes are typically monitored by reflectivity or transmissivity measurements, as the change in the effective refractive indexes upon infiltration results in a shift in the position of the Bragg diffraction peak. For the present study, we have employed a porous BS based on alternating SiO2 and TiO2 nanoparticulate layers, deposited by a simple spin-coating process at 2000 rpm, followed by 15min calcination steps at 450 8C after every bilayer deposition (a representative scanning electron microscopy (SEM) image and reflectance spectrum can be found in Fig. S1 of the Supporting Information). The obtained multilayered films are then laterally patterned by selective etching with a patterned mask put in conformational contact with the film surface, generating an array of nine 3mm 3mm squares. The squares are separately functionalized with different alkoxysilanes for the attainment of a combinatorial array with distinct surface energy characteristics, generating a proof-of-concept 3 3 array of surface functionalized BS. For this purpose, we incorporated the surface functionalities ethyl (Et), butyl, (Bu), hexyl (Hex), octyl (Oct), CF3(CF2)3(CH2)2–(CF4), CF3(CF2)5(CH2)2–(CF6), and CF3(CF2)7 (CH2)2–(CF8) as well as leaving one nonfunctionalized (NF) square. The effect of surface functionalization on the modulation of the optical properties upon vapor infiltration was effectively probed by environmental spectroscopic ellipsometry, using water-saturated nitrogen gas as probe. While we have employed pixels with different hydrophobicities as a proof of concept, the platform is highly versatile as any chemical or biochemical functionality could be incorporated, in principle, by use of the surface chemistry of silicon and metal oxides. Towards a cost-effective and simple platform for combinatorial measurements of color changes in BS arrays exposed to different saturated atmospheres, we have implemented, for the first time, color imagery analysis as an alternative to the conventional optical spectroscopic probe methods. A very similar approach has been reported for the analysis of colorimetric artificial noses. The vapor exposure experiments were performed in a simple configuration consisting of a sealed chamber containing the sample, which is connected to a solvent inlet. A digital camera and

Visible Colloidal Nanocrystal Silicon Light-Emitting Diode

We herein demonstrate visible electroluminescence from colloidal silicon in the form of a hybrid silicon quantum dot-organic light emitting diode. The silicon quantum dot emission arises from quantum confinement, and thus nanocrystal size tunable visible electroluminescence from our devices is highlighted. An external quantum efficiency of 0.7% was obtained at a drive voltage where device electroluminescence is dominated by silicon quantum dot emission. The characteristics of our devices depend strongly on the organic transport layers employed as well as on the choice of solvent from which the Si quantum dots are cast.

Low-k periodic mesoporous organosilica with air walls: POSS-PMO.

Periodic mesoporous organosilica (PMO) with polyhedral oligomeric silsesquioxane (POSS) air pockets integrated into the pore walls has been prepared by a template-directed, evaporation-induced self-assembly spin-coating procedure to create a hybrid POSS-PMO thin film. A 10-fold increase in the porosity of the POSS-PMO film compared to a reference POSS film is achieved by incorporating ∼1.5 nm pores. The increased porosity results in a decrease in the dielectric constant, k, which goes from 2.03 in a reference POSS film to 1.73 in the POSS-PMO film.

Color from colorless nanomaterials: Bragg reflectors made of nanoparticles

We report herein on a facile and reproducible approach to prepare mesoporous nanoparticle-based distributed Bragg reflectors (DBRs) from a diverse group of metal oxide nanoparticles including SiO2, TiO2, SnO2, and Sb:SnO2. The films prepared, regardless of the composition, and following dispersion and process engineering, demonstrate uniform color and high optical quality over large areas. Not only do the prepared NP DBRs possess high reflectivity but also significant mesoporosity, which opens up the opportunity for introduction of a variety of materials into the pores creating new opportunities in optical and optoelectronic devices. In addition, what also must be highlighted are the added-value properties to the above characteristic of the individual NP materials comprising the DBR such as the electrical conductivity and optical transparency of ATO or the photoconductivity of SnO2, which enable new opportunities in a broad range of fields.

Organic light-emitting diode microcavities from transparent conducting metal oxide photonic crystals.

We report herein on the integration of novel transparent and conducting one-dimensional photonic crystals that consist of periodically alternating layers of spin-coated antimony-doped tin oxide nanoparticles and sputtered tin-doped indium oxide into organic light emitting diode (OLED) microcavities. The large refractive index contrast between the layers due the porosity of the nanoparticle layer led to facile fabrication of dielectric mirrors with intense and broadband reflectivity from structures consisting of only five bilayers. Because our photonic crystals are easily amenable to large scale OLED fabrication and simultaneously selectively reflective as well as electronically conductive, such materials are ideally suited for integration into OLED microcavities. In such a device, the photonic crystal, which represents a direct drop-in replacement for typical ITO anodes, is capable of serving two necessary functions: (i) as one partially reflecting mirror of the optical microcavity; and (ii) as the anode of the diode.

Colloidally Stable Germanium Nanocrystals for Photonic Applications

We report the development of a straightforward synthesis for colloidally stable germanium nanocrystals for use as a solution-processable precursor for the bottom-up fabrication of functional thin films. SiO(2)-embedded germanium nanocrystals are produced by the reductive thermal processing of sol-gel glasses derived from mixtures of tetraethoxyorthogermanate (TEOG) and tetraethoxyorthosilicate (TEOS), and free-standing germanium nanocrystals are liberated from the encapsulating silicon dioxide through sequential chemical etching. The applicability of these germanium nanocrystals as a solution-processable thin film precursor is demonstrated by the fabrication of high refractive index thin films.

Selectively Transparent and Conducting Photonic Crystals

[*] Prof. N. P. Kherani, Dr. A. Chutinan The Edward S. Rogers Sr. Department of Electrical and Computer Engineering University of Toronto 10 King’s College Road, Room GB254B Toronto, ON M5S 3G4 (Canada) E-mail: kherani@ecf.utoronto.ca Prof. G. A. Ozin, D. P. Puzzo, L. D. Bonifacio Materials Chemistry Research Group, Department of Chemistry University of Toronto 80 St. George Street, Toronto, ON M5S 3H6 (Canada) E-mail: gozin@chem.utoronto.ca P. G. O’Brien Department of Materials Science and Engineering University of Toronto 184 College Street Room 140, Toronto, ON M5S 3E4 (Canada)

Distributed Feedback Lasing from a Composite Poly(phenylene vinylene)-Nanoparticle One-Dimensional Photonic Crystal

Nanoparticle one-dimensional photonic crystals exhibit intense, broadband reflectivity coupled with a unique mesoporosity. The latter property allows for infiltration of the one-dimensional photonic crystal with functional materials, such as emitting polymers, which in turn can lead to the fabrication of composites whereby the emitters emission can be modulated by the photon density of states of the photonic crystal. We exploit this interaction in order to produce efficient distributed feedback lasing from a composite poly(phenylene vinylene)-infiltrated nanoparticle one-dimensional photonic crystal.

Silicon Nanocrystal OLEDs: Effect of Organic Capping Group on Performance

The synthesis of highly luminescent, colloidally-stable and organically-capped silicon nanocrystals (ncSi) and their incorporation into a visible wavelength organic light-emitting diode (OLED) is reported. By substituting decyl chains with aromatic allylbenzene capping ligands and size-selecting visible emitting ncSi, superior packing density, enhanced charge transport, and an improved photoluminescence absolute quantum yield of the ncSi is obtained in the active layer of an OLED.