Toward Precision Deposition of Conductive Charge‐Transfer Complex Crystals Using Nanoelectrochemistry

Abstract

The lack of understanding for precise synthesis and assembly of nano-entities remains a major challenge for nanofabrication. Electrocrystallization of a charge-transfer complex (CTC), tetrathiafulvalene bromide (TTF)Br, is studied on micro/nanoelectrodes for precision deposition of functional materials. The study reveals new insights into the entire CTC electrocrystallization process from the initial nanocluster nucleation to the final elongated crystals with hollow ends grown from the working electrode to the neighboring receiving electrode. On microelectrodes, the number of nucleation sites is reduced to one by lowering the applied overpotential or precursor concentration. Certain current-time transients exhibit significant induction periods prior to stable nucleus growth. The induction regime contains small fluctuating current spikes consistent with stochastic formation of precritical nanoclusters with lifetimes of 0.1-30 s and sizes of 20-160 nm. Electrochemical analyses further reveal rate, size distribution, and formation/dissipation dynamics of the nanoclusters. Crystal growth of (TTF)Br is further studied on triangular nanoelectrode patterns with thickness of 5-500 nm, which shows a mass-transfer-controlled process applicable for precision deposition of functional (TTF)Br crystals. This study, for the first time, establishes CTC nanoelectrochemistry as a platform technology for precise deposition of conductive crystal assemblies spanning the source and drain electrode for sensing applications.

Microelectrode‐enabled Electrocrystallization of Cobalt TCNQ Complex for Gas Sensing

“…The fit with the Fletcher model indicates that the electrocrystallization of Co‐TCNQ at V=100 mV is limited by interfacial electrokinetic [15] . The i‐t transients from repeated experiments under the same conditions at V=100 mV (Figure S3) display similar slopes of a single nucleation event but with wide‐ranging induction times of 2–2,000 s, consistent with the stochastic nature of CTC nucleation [2w,17] . In some instances, no Co‐TCNQ nucleation was observed during the entire period of our experimental window of 3,600 s. Ex‐situ SEM imaging with elemental mapping (Figure S4) shows the presence of C, N, Co, and traces of O, consistent of the presence of Co‐TCNQ on the microelectrode surface.…”

“…Figure 3c summarizes the morphological changes during Co‐TCNQ electrocrystallization on the Pt microdisk electrode. The hollow crystal morphology, also observed in TTF‐Br electrocrystallization, has been attributed to the diffusion flux gradient across a growing crystalline face where it is the highest at the circumference than the center [2l,m,w,24] …”

“…The $${{I}_{stripping}}$$ process is associated with the dissolution of an electroactive material from the electrode surface. The appearance of a crossover current generally indicates the nucleation and growth of conductive materials [2w] . The $${{I}_{red}}$$ process is attributed to the reduction of TCNQ to TCNQ⋅ − .…”

“…Pt microdisk electrodes with 10 μm in diameter used as the working electrode, Ag/AgCl reference electrode, and Pt wires of 0.5 and 1 mm in thickness used as the counter electrode were purchased from CH Instruments. The microdisk electrode was polished and rinsed with deionized water and MeCN before each experiment following our previously established procedure [2w] . Solutions were prepared from TCNQ (0.8 mM) and Co(NO 3 ) 2 • 6H 2 O (20 mM) in MeCN.…”

“…The scalability issue remains a major obstacle preventing the realization of the full commercial potential of diverse nanomaterials. Our previous work on tetrathiafulvalene bromide (TTF‐Br) crystals has found the microelectrode technique to be a valuable electroanalytical tool for understanding the nucleation and crystal growth of TTF‐Br crystals [2w] . In this study, we perform electroanalysis of Co‐TCNQ nucleation and crystallization on a microdisk electrode using cyclic voltammetry (CV) and chronoamperometry.…”