Two-Dimensional Colloidal PbS Nanosheet Synthesis and Characterization

Jonathan Newman1, Haitao Zhang2, and Frank Wise2 1Department of Material Science and Engineering, Cornell University, Ithaca, NY 14850 2Department of Applied Engineering and Physics, Cornell University, Ithaca, NY 14850

Introduction

Experimental Method

Thickness Control

Surface Modifier

Aliquot Studies

Conclusion

Future Work

Nanosheets (NS) 1-Dimensional quantum confinement Superior charge carrier mobility Lead sulfide nanosheets Near infrared absorption peak Large Bohr radius ~20 nm Synthesis Lead source:

Pb(acetate) 23H2O + OA ! Pb(OA)2 + acetic acid + H2O Sulfur source: Thioacetimide (TAA) Acetic acid Oriented attachment 1. PbS nanoparticles form 2. Ligands attach to surfaces facets 3. Oriented attachment of facets 4. Anisotropic growth to NS

100 nm

Switch lead source - lead (II) oxide - eliminate acetic acid Solvent is diphenyl ether (DE)

Switch sulfur source to bis(trimethylsilyl)sulfide (TMS) TMS is more reactive than TAA

Solvent is trioctylphosphine (TOP)

PbS NS synthesis hot injection colloidal method cheap and easy to manipulate 1. Lead source: PbO, oleic acid and DE injected into flask 2. Heated to 120 oC 3. Chlorine-containing compound is injected into the solution 4. Sulfur source: TOP, Dimethylformamide, TMS added to solution at

reaction temperature for reaction time 5. Cool to room temperature and store in toluene

20 min 40 min 60 min

Reaction Time Absorption Peak PL Peak 20 min 1380 nm 1450 nm 40 min 1520 nm 1560 nm 60 min 1610 nm 1650 nm

Red Shift

20 min

40 min 60 min

Reaction Temp Absorption Peak PL Peak 80 C 1185 nm 1255 nm 90 C 1380 nm 1455 nm

100 C 1540 nm 1600 nm

Red Shift

PbS PbS CdS Cd(OA)2

Purpose Reduce surface trap states Surface trap states Quench PL emission Damage charge transportation Results Improved colloidal solubility and

stability PL red shift shows etching of PbS Dramatic quantum yield increase FL lifetime increase proportional to

PL intensity PL quenching reduction

STEM images show visual etching Cubic PbS lattice (left) replaced by

darkened CdS hexagonal lattice on nanosheet surface (right)

0 h 2 h

12 h

36 h

PL QY: 5%

PL QY: 11%

0 h

2 h 12 h 36 h

PL Increasing Treatment Time

Fluorescence Lifetime Increasing Treatment Time

Before Cd(OA)2 treatment After Cd(OA)2 treatment (12h)

1 nm

cubic PbS lattice

Visible etching on PbS nanosheet edge

1 nm cubic PbS lattice

hexagonal CdS lattice

New synthesis Developed more reproducible PbS NS synthesis method Shape control Possible through oleic acid and temperature variations Thickness control Highly tunable nanosheet thickness Reaction time variations Reaction temperature variations Surface modification Use of cadmium oleate improved optical properties and

dispersibility

Increase monodispersity of thickness Produce narrower photoluminescence and absorption peaks Formation mechanism Continued research to further understand Understanding will lead to improved products Optoelectronic devices

Shape Control

200 nm

Pb : HOA = 1 : 4.4; 90 oC Pb : HOA = 1 : 2.2; 90 oC Pb : HOA = 1 : 2.2; 80 oC

200 nm 200 nm

Shape Control From nanobelts to

nanosheets with temperature and oleic acid concentration

Synthesis Conditions (Pb:HOA, Temp.)

Approx. Dimensions (nm)

1 : 2.2; 80 oC 150 x 20 1 : 2.2; 90 oC 200 x 50 1 : 4.4; 90 oC 200 x 100

Temperature Increase HOA Concentration Increase

500 nm

Poor reproducibility due to acetic acid

Depiction of Oriented attachment

Thickness Determines relative optical properties and tunablility for more accurate device control

Reaction time The absorption peaks are

determined with secondary differentials

Going from 20 minutes to 60 minutes there is a clear red shift in the both peaks

Indication that NS are becoming thicker and thicker with increased reaction time

Optical characterization of PbS nanosheets synthesized at Pb:HOA = 1 : 2.2, 90 oC for different times: I. 20 min; II. 40 min; III. 60 min

Reaction temperature Red shift, again, indicating the

nanosheets are getting thicker and thicker

Monodispersity proportional to the width of the PL peaks 90 C NC more monodisperse

than 80 C NS (90 C peak is narrower)

Optical characterization of PbS nanosheets synthesized at Pb:HOA = 1 : 2.2, 20 min for different temperatures: 1. 80 C; II. 90 C; III.100 C

Vacuum for 20 min Vacuum for 1 hour

9 min

150 nm

200 nm 200 nm

200 nm

200 nm 200 nm

Sudden appearance of NS between 4 and 5 minutes No other dramatic NS growth Supports oriented attachment as nanoparticles form first and

appear to instantly form nanosheets 2-D skeleton formed followed by growth in thickness over time

8 min

7 min 6 min

5 min 4 min

100 C

90 C 80 C

90 C 80 C 100 C Absorption PL

Absorption PL

Etching of PbS surface through cation exchange