MANIPULATION OF IONS, ELECTRONS, AND PHOTONS IN 2D MATERIALS BY ION INTERCALATION

Abstract

2D materials have attracted tremendous attention due to their unique physical and

chemical properties since the discovery of graphene. Despite these intrinsic properties,

various modification methods have been applied to 2D materials that yield even more

exciting results. Among all modification methods, the intercalation of 2D materials

provides the highest possible doping and/or phase change to the pristine 2D materials.

This doping effect highly modifies 2D materials, with extraordinary electrical transport as

well as optical, thermal, magnetic, and catalytic properties, which are advantageous for

optoelectronics, superconductors, thermoelectronics, catalysis and energy storage

applications. To study the property changes of 2D materials, we designed and built a

planar nanobattery that allows electrochemical ion intercalation in 2D materials. More

importantly, this planar nanobattery enables characterization of electrical, optical and

structural properties of 2D materials in situ and real time upon ion intercalation. With this

device, we successfully intercalated Li-ions into few layer graphene (FLG) and ultrathin

graphite, heavily dopes the graphene to 0.6 x 10^15 /cm2, which simultaneously increased its conductivity and transmittance in the visible range. The intercalated LiC6 single crystallite achieved extraordinary optoelectronic properties, in which an eight-layered Li intercalated FLG achieved transmittance of 91.7% (at 550 nm) and sheet resistance of 3 ohm/sq. We extend the research to obtain scalable, printable graphene based transparent conductors with ion intercalation. Surfactant free, printed reduced graphene oxide transparent conductor thin film with Na-ion intercalation is obtained with transmittance of 79% and sheet resistance of 300 ohm/sq (at 550 nm). The figure of merit is calculated as the best pure rGO based transparent conductors. We further improved the tunability of the reduced graphene oxide film by using two layers of CNT films to sandwich it. The tunable range of rGO film is demonstrated from 0.9 um to 10 um in wavelength. Other ions such as K-ion is also studied of its intercalation chemistry and optical properties in graphitic materials.

We also used the in situ characterization tools to understand the fundamental properties

and improve the performance of battery electrode materials. We investigated the Na-ion

interaction with rGO by in situ Transmission electron microscopy (TEM). For the first

time, we observed reversible Na metal cluster (with diameter larger than 10 nm)

deposition on rGO surface, which we evidenced with atom-resolved HRTEM image of

Na metal and electron diffraction pattern. This discovery leads to a porous reduced

graphene oxide sodium ion battery anode with record high reversible specific capacity

around 450 mAh/g at 25mA/g, a high rate performance of 200 mAh/g at 250 mA/g, and

stable cycling performance up to 750 cycles. In addition, direct observation of

irreversible formation of Na2O on rGO unveils the origin of commonly observed low 1st

Columbic Efficiency of rGO containing electrodes. Another example for in situ

characterization for battery electrode is using the planar nanobattery for 2D MoS2

crystallite. Planar nanobattery allows the intrinsic electrical conductivity measurement

with single crystalline 2D battery electrode upon ion intercalation and deintercalation

process, which is lacking in conventional battery characterization techniques. We

discovered that with a “rapid-charging” process at the first cycle, the lithiated MoS2

undergoes a drastic resistance decrease, which in a regular lithiation process, the

resistance always increases after lithiation at its final stage. This discovery leads to a 2-

fold increase in specific capacity with with rapid first lithiated MoS2 composite electrode

material, compare with the regular first lithiated MoS2 composite electrode material, at

current density of 250 mA/g.