newman, jonathan poster
DESCRIPTION
My senior thesis poster.TRANSCRIPT
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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