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