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Two DimensionalTransition Metal DichalcogenidesRICARDO VIDRIO, Physics Department, UCSB
RYAN NEED, STEPHEN WILSON, Materials Department, UCSB
Two dimensional materials, like graphene and TMDCs,have a planar structure ideal for flexible electronics.
Graphene Carbon atoms with hexagonal bonding; metallic conductivity
MX2M = transition metalX = chalcogenide
MM’X4
M = transition metal #1M’ = transition metal #2X = chalcogenide
Transition metal dichalcogenides (TMDCs)Two or more types of atoms with octahedral bonding; semi-conductivity
Drawbacks
All necessary precursors must be sealed in quartz ampoules
Reactions are slow
Trial and error process
Experimental Parameters
Thot = 1000° C
Tcold = 900 °C
Pressure = 5x10-5 millibar
We used chemical vapor transport reactionsto try to grow NbIrTe4, a ternary TMDC.
This type of reaction is commonly used to grow binary TMDCs (e.g. WTe2, NbSe2)
Initial powders
Intermediates formed by powder reacting with a halogen transport agent Crystals
(hopefully)
Our CVT reactions resulted either inreacted powder or small crystals.
powder
crystals
If we got powder, we would:
(1) Remove it from the ampoule
(2) Grind it with a mortar and pestle
(3) Use x-ray diffraction to determine the phases present and their relative amounts
If we got crystals, we would:
(1) Remove them from the ampoule carefully
(2) Use electron microscopy to determine the habits and morphology
(3) Use energy-dispersive x-ray spectroscopy to determine the chemical composition
XRD on our powder showed inconsistencybetween the nine batches we attempted.
XRD patterns from four select batches. The last two patterns are from nominally identical batches.
Constructive interference of x-rays diffracted from a series of parallel planes within a crystal lattice gives rise to peaks.
Chemical and electrical analysis on our crystalsrevealed them to be pure tellurium.
Tellurium
NbIrTe4
Previous chemical vapor transport reaction studies have been primarily focused on binary systems. These have fewer possible products.
The added complexity of a third elementlead to these inconsistent and undesired results.
The addition of this third element, M’, into our reaction greatly increases the number of possible products and inhibits formation of pure MM’X4.
W(s) + 2Br2(g) ⇌ WBr4(g)
Te(s) ⇌ Te2(g)
Hot end (vaporization)
WBr4(g) + Te2(g) ⇌ WTe2(s) + 2Br2(g)
Cold end (deposition)
Nb(s) + 2Br2(g) ⇌ NbBr4(g)
Te(s) ⇌ Te2(g)
Hot end (vaporization)
NbBr4(g) + Te2(g) ⇌ NbTe2(s) + 2Br2(g)
Cold end (deposition)
NbBr4(g) + 2Te2(g) ⇌ NbTe4(s) + 2Br2(g)Ir(s) + 2Br2(g) ⇌ IrBr4(g)
3IrBr4(g) + 4Te2(g) ⇌ Ir3Te8(s) + 6Br2(g)
NbBr4(g) + IrBr4(g) + 2Te2(g) ⇌ NbIrTe4(s) + 4Br2(g)
We went back to the drawing boardand began trying a new form of growth reaction.
Tellurium
Niobium
Iridium We placed tellurium in the bottom of a sealed ampoule in hopes that when heated to 1000°C, the other powders would mix into the molten tellurium.
Tellurium has a considerably lower melting point (450°C) than either niobium or iridium (~2400°C).
In fact, the powders did not mix well, likely because of the confining geometry of the ampoule.
Instead, some small amount of niobium and iridium dissolved at the interface of this molten tellurium and crystal grew.
Results from our first batch using this layeringrevealed mm-sized crystals of NbIrTe4.
Crystals had flat, plate-like geometrythat flaked easily when handled.
EDS revealed a 1.2:1:4 ratiostrongly indicating we had NbIrTe4.
• Repeat and hopefully replicate growth of NbIrTe4 crystals.
• Determine whether the layering of niobium and iridium is necessary? Is there a optimum order for layering?Would mixing those element powders be better?
• Optimize growth conditions (i.e. soak temperature, ramp rates)for NbIrTe4 crystal formation.
• Once growth of NbIrTe4 is optimized, we will move on to previously unstudied forms of two dimensional transition metal dichalcogenides, such as ZrIrTe4, that may harbor unique properties.
Moving forward, we will…
This project was partially supported by the LSAMP program of theNational Science Foundation under Award no. DMR-1102531 and by the MRSECProgram of the National Science Foundation under Award No. DMR- 1121053
I also acknowledge the contribution that Ryan Need has placed forward on this project. His mentorship and patience proved invaluable throughout the entire summer. I also want to thank Prof. Stephen Wilson for allowing me to research in his lab this summer.
Acknowledgements and references:
1. http://www.nature.com/nnano/journal/v7/n11/images/nnano.2012.205-i1.jpg
2. Schmidt, Binnewies, Glaum, Schmidt. Adv. Topics on Cryst. Growth. InTech, (2013) Chp. 9, 227-305.
3. Brown, B. E. The crystal structures of WTe2 and high-temperature MoTe2, Acta. Cryst. 20 (1966) 268.
4. http://www.ualberta.ca/~pogosyan/teaching/PHYS_130/FALL_2010/lectures/lect36/lecture36.html
5. http://chemwiki.ucdavis.edu/Analytical_Chemistry/Instrumental_Analysis/Diffraction/Powder_X-
ray_Diffraction
6. http://www.microscopy.ethz.ch/bragg.htm
• Repeat and hopefully replicate growth of NbIrTe4 crystals.
• Determine whether the layering of niobium and iridium is necessary? Is there a optimum order for layering?Would mixing those element powders be better?
• Optimize growth conditions (i.e. soak temperature, ramp rates)for NbIrTe4 crystal formation.
• Once growth of NbIrTe4 is optimized, we will move on to previously unstudied forms of two dimensional transition metal dichalcogenides, such as ZrIrTe4, that may harbor unique properties.
Moving forward, we will…
Percent yield of powder NbIrTe4 present among all of our ampoules
Batch Number Powder Ratio Percent NbIrTe4 Transport Agent
2 1:1:4.3 97.7 Br2
3 1:1:4 93.4 none
4 1:1:8 0 none
5 1:1:4 0 none
6 1:1:4 0 I2
7 1:1:4.3 0 none
8 1:1:4.3 5.4 Br2
9 1:1:4.3 73.7 I2
Amounts of powder used in experimentationAll mass amounts are in mg
Element Mass with excess
tellurium (1:1:4.3)
Mass without excess
tellurium (1:1:4)
Mass with excess
tellurium (1:1:8)
niobium (Nb) 111.4 116.8 782
iridium (Ir) 230.5 241.6 147
tellurium (Te) 658.0 641.6 71.1