Charge transport in nanopatterned PbS colloidal quantum dot arrays

Abstract

In this thesis, we study charge transport in nanopatterned arrays of PbS colloidal quantum dots using conventional two-probe measurements and an integrated charge sensor. PbS dots are synthesized in solution with an organic ligand or cap that serves to passivate the surface of the dot, provide a tunnel barrier as well as colloidal stability. These dots can self assemble into an array as the solvent evaporates from a drop of solution on a surface. The self-assembled arrays can be thought of as tunable artificial solids, where the coupling between the dots can be tuned by changing the ligand. Using electron beam lithography followed by a lift-off process, we develop a novel technique to nanopattern these arrays and present the first colloidal quantum dot arrays of nanoscale dimensions. Nanopatterning makes it possible to study the electrical properties intrinsic to the dots unimpeded by macroscopic defects, such as cracking and clustering that typically exist in larger-scale arrays. We find that the electrical conductivity of the nanoscale films is higher than that of drop-cast, microscopic films made of the same type of dot. We achieve unprecedented versatility in integrating the patterned films into device structures, which will be valuable both for studying the intrinsic electrical properties of the dots and for nanoscale optoelectronic applications. From two-probe measurements on the nanopatterned arrays that are approximately 15 dots wide, we observe large noise in the current as a function of time. The noise is proportional to the current when the latter is varied by applying source-drain or gate voltage in a field-effect structure or when changing temperature. Owing to the small number of current paths in the system, we often observe telegraph switching, and find that the off times follow non-poissonian statistics. We show that the results can be understood in terms of a model in which a quasi-one-dimensional percolation path is turned on and off, by charging of a dot along the path. Long organic ligands lead to highly resistive colloidal quantum dot arrays, making the low bias regime inaccessible with conventional two-probe measurements. We use an integrated charge sensor to study transport in the low bias regime as a function of the coupling between the dots. We present transport measurements on butylamine and oleic acid capped PbS dots. The resistances measured are the highest measured for colloidal quantum dots. For the native oleic acid ligand, and weak coupling between the dots, the conduction mechanism is nearest neighbor hopping, and the conductance is simply activated. At low source-drain bias voltages, the activation energy is given by the energy required to release a carrier from a trap state plus the activation over barriers resulting from site disorder. The barriers from site disorder are eliminated with a sufficiently high source-drain bias. For the shorter ligand, which gives stronger coupling, the data are consistent with Mott's variable range hopping as the conduction mechanism.