Tailored Graphene Based Assemblies E Experiment i t and d Th Theory K. Vinodgopal, M. Wu, D. Taylor, I. Bondarev and B. Vlahovic Departments p of Physics y and Chemistryy North Carolina Central University

CREST External Advisory Board Meeting, NCCU, November 18th, 2011

Project Scope Designing single layer graphene-metal assemblies using ultrasound

Theoretical determination of electron transport mechanisms h i and d carrier mobility

Integrated Approach to New Graphene Based Materials

Applications in solar cells, f l cells, fuel ll sensors

GO Surface Initiated Polymerization off Polymer P l Brushes

v1 v2

Slide 2 v1

vinod, 4/4/2011

v2

vinod, 4/4/2011

How to use chemistry to k graphene h h t ? make sheets? Hummers Method

Graphene oxide (GO)

Soluble in polar solvents.

Designing Graphene/Metal Assemblies Chemical Reduction

High Frequency Ultrasound

Graphene oxide (GO) + metal salt

Reduced Graphene Oxide (RGO) + metal nanoparticle

Acoustic cavitation +

Ti Time

Tmax

 m   1   T0    v 

-

Initial bubble Bubble collapse

Tmax= maximum bubble T To = solution T Pm = Liquid P (Phydrostatic) (Pacoust + P0) Pv = P C in i p the th bubble b bbl γ= before collapse C v

The power of cavitation Bubble Implosion - “near adiabatic” Leads to localised areas of high T and P Tmax of the order of 5000 - 15000 K Pmax of the order of 100 - 1000 atm Large gradients of T P, T, P shear h Consequences of these extreme conditions Radical generation - Sonochemistry Light emission - Sonoluminescence Shock waves, microjetting, microstreaming, i i shear h fforces, etc.

Reactive species

HIGH T, P

Rapid motion and mixing

Why ultrasound ? Simultaneous sonochemical reduction of metal salts in solution to yield RGO/metal nanoparticle assemblies – better accomplished with HF transducers (200-300 kHz). Ultrasound method p produces exfoliated RGO sheets in aqueous solution- better accomplished ) with LF transducers ((20 kHz).  Synthesis of large mm sized graphene sheets (LGS).

Low frequency 20 kHz ultrasound transducer Ti i Titanium horn-sonotrode h d

High frequency 211 kHz ultrasound transducer

RGO + Au simultaneous reduction

Sonolytic Design of Graphene-Au Nanocomposites. Simultaneous and Sequential Reduction of Graphene O id and Oxide d Au(III) A (III) J. J Phys. Phys Chem. Chem Lett. Lett 2010, 2010 11, 1987 1987-1993. 1993

RGO w/Au

Intensitty

80000 785 nm

a: RGO b: RGO + Au (seq) c: RGO + Au (sim)

60000

c 40000

20000 1200

b a 1300

1400

1500

1600

Raman shift,cm

1700

1800

-1

 Surface enhancement only seen in bi-layered and few layered

graphene and with Au particles grown in situ on the graphene surface.  Dependent on density of coverage .  CT from graphene to gold -aa band-gap band gap dependence. dependence

Dual frequency Ultrasound arrangement 20 kHz Ti horn transducer in pulsed mode for exfoliation (4 ms pulse width and12 ms pulse separation, separation power = 11.0 W)

GO + metal salt

211 kHz CW transducer for chemical reduction, power = 6.2 W

Research Plan 1 Synthesis 1. S h i off LGO (large (l GO) using i ultrasound l d

50m

Synergy with theory: Calculations of Casimir forces in graphene and comparison with shear forces generated by acoustic waves.

Research plan (continued) 2. RGO/Au nanocomposites .  One pot sonochemical synthesis and routine characterization by UV-VIS, TEM, Raman and FTIR-ATR.  Raman spectroelectrochemical studies on these thin films as a function of the nanoparticle coverage, exciting laser wavelength, and bias potential.  Theoretical analysis of electronic response functions for confined graphene systems with metal nanoparticles.  Application of these composites (with optimized properties) as substrates for sensing chemical species at low concentrations by Raman spectroscopy.

Surface Initiated Polymerization of Styrene Brushes from Graphene Sheets

Graphene-Oxide (b)

Graphene-Ruthenium Nanocomposite p

Enabling the Next Generation of Organic Solar Cell Devices

Research Plan RGO with polystyrene brushes and light harvesting (Ru) chromophore.  Grafting of polystyrene brushes on the RGO surface.  Attachment Att h t off Ru R chromophores h h att endd off brush. b h  Varying the length of the polymer brushes and their density on the surface to investigate the mechanisms for energy transfer between chromophores on adjacent polymer strands and from chromophore to graphene substrate.  Theoretical calculations on the potential barriers with respect to such energy transfer.

Surface Initiated Polymerization w/ stryene

Students Graduate Students: Khoa Le -Ist year Masters Arun Sapkota- Ist year Masters Lokendra Chand – 2ndd year Masters U d Undergraduate: d t Najah Salleh (senior)