Engineering electron and hole wires in 2D materials through polar discontinuities

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1 THEORY AND SIMULATION OF MATERIALS Engineering electron and hole wires in 2D materials through polar discontinuities Marco Gibertini, Giovanni Pizzi, Nicola Marzari US - EU Workshop on 2D Layered Materials and Devices Arlington (USA) April 22, 2015

2 Who are we? Swiss Federal Institute of Technology (Lausanne, Switzerland) Prof Nicola Marzari Dr Matteo Cococcioni Dr Oliviero Andreussi Dr Giovanni Borghi Dr Marco Gibertini Dr Ngoc Linh Nguyen Dr Giovanni Pizzi (Theory and Simulation of Materials) Dr Nicolas Mounet Dr Ivano Castelli Dr Nicola Colonna Dr Aris Marcolongo Dr Martin Uhrin Dr James Moraes de Almeida Dr Nicola Varini Andrea Cepellotti Anand Chandrasekaran Daniele Dragoni Gianluca Prandini

3 What about MARVEL? SWISS CENTER FOR COMPUTATIONAL DESIGN AND DISCOVERY OF NOVEL MATERIALS Nicola Marzari, IMX, EPFL EPFL-ETHZ-UNIBAS-UNIFR-UNIGE-USI-UZH-IBM-CSCS-EMPA-PSI The goal: Accelerated design and discovery of novel materials via a materials informatics platform of high-throughput quantum simulations 12-year timespan ( ), and 11 Swiss Institutions: 2 Federal, 5 Cantonal, 1 Industrial, 1 Supercomputing Centre, 2 Experimental Labs First phase ( ): 25 PIs, 34.4M CHF (18M SNSF, 6.6M EPFL, 9.8M others)

4 What do we UNDERSTAND-PREDICT-DESIGN novel materials and devices from first-principles quantum simulations Examples for 2D materials: thermal and electrical transport k (W / mk) T = 300 K 0% 13 C 1.1% 13 C L (µm)! e ph (") n (10 13 cm 2 ) T (K) (c) Theory n (10 13 cm 2 ) T (K) (d) Experiment (Efetov & Kim) G. Fugallo, A. Cepellotti et al., Nano Letters 14, 6109 (2014) A. Cepellotti et al., Nat. Commun. 6, 6400 (2015) C.-H. Park, N. Bonini et al., Nano Letters 14, 1113 (2014) T. Sohier et al., Phys. Rev. B 90, (2014)

5 Design: novel devices for energy harvesting and microelectronics

6 Engineering polar discontinuities in 2D Polar discontinuity across interfaces that naturally arise out of a single parent crystal using experimentally well established 2D materials Functionalizations Nanoribbons Grain/phase boundaries N.C. Bristowe et al., Phys. Rev. B 88, (2013) MG, G. Pizzi, N. Marzari, Nature Communications 5, 5157 (2014)

7 Functionalizations

8 Selective functionalization Covalent functionalization with H or F changes bulk polarization BNH 2 BN P BN = e a λ pol = e 3a P BNH2 = 2e 3a λ pol = e 3a BN P BN = e a By selective functionalization we introduce a polar discontinuity

9 Zigzag interfaces - LDOS Energy (ev) Position (Å) Electric fields are present on both sides The system becomes metallic Charge transfer from top VB to bottom CB Electrons and holes are localized on different interfaces MG, G. Pizzi, N. Marzari, Nature Communications 5, 5157 (2014)

10 Zigzag interfaces - Free charge Integrate the LDOS to get the charge density of free carriers Charge density (e/a) λ P λ F λ tot Total width (Å) The total free charge λ F (per unit length) increases with size Asymptotic perfect screening of polarization charge λ P = P 0 BNH 2 P 0 BN Total charge at the interface λ tot = 0 ( E BN + E BNH2 )

11 Functionalization of Graphene/BN Planar graphene/boron nitride heterostructures recently realized experimentally Full (instead of selective) functionalization Both functionalized materials are insulating Polar discontinuity at the interface Free electrons/holes appear at the interfaces P. Sutter et al., Nano Letters 12, 4869 (2012) Z. Liu et al., Nat. Nanotechnol. 8, 119 (2013) L. Liu et al., Science 343, 163 (2014) Y. Gong et al., Nat. Commun. 5, 4193 (2014)

12 Nanoribbons

13 Discontinuity with vacuum: nanoribbons Vacuum can be considered as an insulator with zero polarization Cutting a polar 2D material into a nanoribbon creates a polar discontinuity Potential energy (ev) MoS An electric field appears inside the nanoribbon... free electrons...and drives an insulator-to-metal transition free holes Position (Å) Possible materials: BN, MoS 2 and all group-vi TMDs, etc. Completely different origin with respect to edge states in graphene F. Güller,et al., Phys. Rev. B 87, (2013) MG and N. Marzari, to be submitted (2015)

14 Grain and phase boundaries

15 Inversion domain boundaries Inversion symmetry is!"#$%& '()$#*+&"', broken in heteroatomic honeycomb lattices4 '-.$&+//"% 01) ", 2'3 Inversion domain boundaries along the zigzag direction observed in MoS2 samples The largest polar discontinuity is expected to occur across these boundaries Metallic boundary states appear to screen polarization charges PRL 113, (2014) X. Zou et al., Nano Lett. 13, 253 (2013) MG and N. Marzari, to be submitted 10 PHYSICAL REVIEW L W. Zhou et al., Nano Lett. 13, 2615 (2013) H. Liu et al., Phys.!"#$!" #$%!"#$ %&''&() Rev. Lett. 113, (2014) 56% &'() *&+(,-

16 Phase-engineered interfaces in gr-iv TMDs The most stable phase for group-iv TMDs is the 1T, while the 2H is metastable 2H trigonal prismatic coordination 1T octahedral coordination insulating TiS 2 TiS 2 TiSe 2!! TiSe 2 ZrS 2 ZrS 2 ZrSe 2 ZrSe 2 Polarization: HfS 2 HfS 2 HfSe 2 HfSe 2 P 2H = 4 e P 1T =0 polar 3 a discontinuity

17 Applications 1D reconfigurable metallic channels for new-generation ultra-thin electronics and spintronics Integrated ultra-thin solar cells. Bulk insulating areas as active regions Built-in electric field to split electrons and holes 1D channels to remove e/h from active region and store energy

18 Summary Honeycomb lattices offer a rich playground to engineer polar discontinuities and e/h wires in a variety of materials Functionalizations Nanoribbons Grain/phase boundaries Well-established 2D materials: BN, group-vi and IV TMDs (MoS 2,...), graphene/bn interfaces Promising applications in electronics/spintronics and solar-energy harvesting