Combinatorial approaches toward patterning nanocrystals
Abstract: A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces. The process results in spatially and chemically distinct interaction sites on a single substrate. Nanocrystal assembly occurs with a high selectivity on just one type of site. We report on the production of binary, tertiary, and quatemary patterns of nanocrystals. We highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, we investigate the assembled structures using photoluminescence and absorption spectroscopy.
Summary (2 min read)
- The process is based on lithographic patterning of organic monolayers that contain a photolabile protection group and are covalently bound to SiO2 surfaces.
- The particles and to prevent formation of bulk material, any p terning approach must be carried out at low temperatures in nonchemically hostile environments.
- The chemistry involved the selective assembly of nanocrystals is different than the case of molecules, and these differences are highlig and discussed.
A. General information
- All chemicals used were of analytical grade or of high purity available and obtained from Aldrich, Fisher, Acro Gelest, and Boehringer Mannheim.
- Spectra were taken with a home-built spectrometer using 473.6 nm line of an argon–krypton laser~coherent! for excitation.
- Images were taken with a Cambridg 360 scanning electron microscope operated at 15 kV.
- B. Preparation of nitroveratryloxycarbonyl–glycine „NVOC–GLY… Synthesis of the compound NVOC–GLY has been m tioned before12 but has not been described in detail.
D. Pt nanocrystals
- The volume of the black ganic layer was reduced to about 10 mL by rotary evapo tion.
- The particles were precipitated by adding 400 mL a etone and storing at24 °C overnight.
- Images of the samples revealed average metal core diameter of 2.560.5 nm.
- The concentra tion used in experiments with the lithographically pattern substrates corresponded to an optical density of 0.5 at 1 path length at 700 nm.
E. Epitaxially grown CdSe/CdS-core/shell structured nanocrystals
- The synthesis of dodecylamine stabilized CdSe/C core/shell structured NCs has been described in literature.
- Co responding to their emission maxima the particles had a age CdSe core diameters of 2.4 and 3.7 nm and a CdS thicknesses of about 1.5 and 1.0 ML, respectively.
- As co pared to Rhodamine 560, the fluorescence quantum y was around 18% for the YNC solution and 8% for the RN solution.
- The particle concentrations of the solutions the authors u for treating the substrates corresponded to an optical den of 0.5 at 350 and 430 nm for the YNCs and the RNCs a cm path length, respectively.
F. Preparation of photosensitive glass and Si/SiO 2 substrates
- Glass slides and Si wafers were cleaned according to procedure described by Linfordet al.15 Briefly, microcover slides and pieces of silicon wafers were cleaned by trea them first with a mixture of concentrated H2SO4 and 30% H2O2 ~70:30 v/v! at 100 °C for 1.5 h.
- The substrates were wash extensively and stored under 18.2 MV cm water until needed.
- All prepa tions with the light sensitive NVOC group were carried o in a dark-room environment.
G. Lithographic fabrication of patterned nanocrystal arrays
- This long exposure time is due to the low-power UV em sion from the lamp.
- Th amplification of particle binding, where the initially as sembled particles themselves served as a selective subs was repeated once.
- The whole slide was then immersed in solution of YNCs for 45 min~step 2!.
- An amino-functionalized substrate that had been protected with photolabile NVOC group was partly deprotected by irradiation throug mask in the near UV~I!.
- The substrate was then treated with a solution of amine bilized nanoparticles, which assembled on areas of deprotected a groups~II !.
A. Preparation of photosensitive glass and Si/SiO 2 substrates
- Under the conditions the authors chose to functionalize glass Si/SiO2 substrates with amino groups it was found that trialkoxysilane 3-aminopropyltriethoxysilane polymerize and precipitated on the substrate.
- Even the best slides, when probed by S were often characterized by a small amount of submic particulate matter on the substrate.
- This matter appare originated from the surface-functionalization step.
- As m sured by the efficiency of particle binding, the dialkoxys lanes generated a higher coverage of amino sites than monoalkoxysilanes, and were used for most of the exp ments discussed here.
- Solution, the authors could not reac NVOC–Cl with surface-bound amino groups.
B. Lithographic fabrication of patterned nanoparticle arrays
- This pattern could be dev oped with fluorescent markers.
- Selective assembly of particles onto amine-patter substrates works well if the particle surfactants readily dergo ligand exchange with the surface amino groups.
- After the red luminescent particles had been bound, NVOC half of the slide was photodeprotected and the wh substrate was immersed in a solution of YNCs~Fig. 2, step 2!.
- ~corresponding to the similarly labeled quadrants of Fig. 2! the absorption spectra of the quadrants Y/Au and R/Au clearly reveal presence of Au NCs as indicated by the absorption cent at 530 nm.
- U these data, and taking into account the different PL quan yields, the authors calculated that the binding selectivity is arou 93%.
- Amino functionalized substrates that have been bloc with the photolabile protection group NVOC can be used prepare complex, two-dimensional nanoparticle arrays lithographic masking techniques.
- The technique for prod ing these patterns occurs under conditions that are both bient and chemically mild.
- Other techno logically relevant substrates, such as semiconducting p mers, should be relatively straightforward extensions of procedures developed here.
- The authors are currently working on and other aspects relevant to integrating nanocrystal ass blies into microelectronic test devices.
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Q1. What have the authors contributed in "Combinatorial approaches toward patterning nanocrystals" ?
A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals is presented. The authors report on the production of binary, tertiary, and quatemary patterns of nanocrystals. The authors highlight and discuss the differences between nanocrystal/substrate assembly and molecule/substrate assembly. Finally, the authors investigate the assembled structures using photoluminescence and absorption spectroscopy.