1 Tunable Localized Charge Transfer Excitons in a Mixed Dimensional van der Waals Heterostructure

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Tunable Localized Charge Transfer Excitons in a Mixed

Dimensional van der Waals Heterostructure

Mahfujur Rahaman1, Emanuele Marino2,3, Alan G. Joly4, Seunguk Song1, Zhiqiao Jiang2,5, Brian T.

O’Callahan4, Daniel J. Rosen5, Kiyoung Jo1, Gwangwoo Kim,1 Patrick Z. El-Khoury4, Christopher

B. Murray2,5, and Deep Jariwala1

1Department of Electrical and Systems Engineering, University of Pennsylvania, PA 19104, USA

2Department of Chemistry, University of Pennsylvania, PA 19104, USA

3Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Via Archirafi 36, 90123

Palermo, Italy

4 Physical and Chemical Sciences Division, Pacific Northwest National Laboratory, Richland, WA

99352, USA

5 Department of Materials Science and Engineering, University of Pennsylvania, PA 19104, USA

Abstract

Observation of interlayer, charge-transfer (CT) excitons in van der Waals heterostructures

(vdWHs) based on 2D-2D systems has been well investigated. While conceptually interesting,

these charge transfer excitons are highly delocalized and spatially localizing them requires

twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome

via making mixed dimensional vdWHs (MDHs) where one of the components is a spatially

quantum confined medium. Here, we demonstrate the formation of CT excitons in a 2D/quasi-

2D system comprising MoSe2 and WSe2 monolayers and CdSe/CdS based core/shell nanoplates

(NPLs). Spectral signatures of CT excitons in our MDHs were resolved locally at the 2D/single-

NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By

varying both the 2D material, the shell thickness of the NPLs, and applying out-of-plane electric

field, the exciton resonance energy was tuned by up to 120 meV. Our finding is a significant step

towards the realization of highly tunable MDH-based next generation photonic devices.

Introduction

Interlayer excitons (ILXs) are composed of Coulomb bound electron and hole (e-h) pairs

confined in two different spatially separated quantum wells that are coupled together

electronically. Owing to large spatial separation of e-h pairs, ILXs possess much longer lifetimes

(1 – 3 order of magnitude higher) than the direct excitons of individual QWs1,2. This allows ILXs

to be subsequently explored for strongly correlated condensed matter phenomena such as Bose-

Einstein condensates as well as in excitonic, and photonic devices3,4. Experimental observation of

ILXs was first reported in coupled GaAs/AlGaAs QWs and later in various III-V and II-VI QW

heterostructures5. However, the very small exciton binding energy (few meV) of conventional 3D

semiconductor QW heterostructures limited the progress of this field to cryogenic

measurements6.

The recent emergence of both structural as well as electronic variety in 2D materials has

opened new opportunities to study ILXs. Van der Waals heterostructures (vdWHs) composed of

several combinations of distinct 2D materials, especially transition metal dichalcogenides

(TMDCs), allow the formation of ILXs with remarkably high binding energies (100 – 350 meV)7.

Hence, it is possible to observe ILXs in such vdWHs at room temperature (RT), which has made

it an intense research topic in recent years8,9. ILXs formed in 2D/2D systems are generally

delocalized in the 2D plane and require a specific twisting angle between the participating

monolayers to create localized excitons in the 2D landscape10,11. As a result, forming localized

ILXs in a twisted heterobilayer (HB) can function as quantum dot-like (QD) confined potentials

which unlock exciting opportunities towards high-performance semiconducting lasers, single

photon emitters, entangled photon sources, and tunable exotic quantum phases of matters4,12,13.

Despite the recent great efforts of spatially confining ILX in HBs with precisely controlled angles,

imperfection in crystals, challenges with sophisticated sample preparation, and the repulsive

interaction between the confined excitons keep the localization process far from ideal both in

terms of spectral lines and spatial extent14,15.

In this context, mixed dimensional heterostructures (MDH) composed of 2D materials on

one side and 0D or spatially confined materials on the other side can be an attractive option for

the creation of localized ILXs. Due to the van der Waals nature of the interface formed between

2D and 0D or spatially confined materials, MDHs favor similar charge transport phenomena

analogous to all-2D vdWHs, when formed with type-II band alignment16–19. Therefore, it is

predicted that MDHs can also emit ILX-like excitons, which are known as hybrid or charge

transfer (CT) excitons20,21. Additionally, reduced dimensionality of one of the materials can

introduce arbitrary spatial and energy confinement as well as additional degrees of freedom at

the interfaces to tune electronic properties of MDHs20. Hence, in contrast to delocalized ILXs in

all-2D vdWHs, CT excitons formed in MDH heterointerfaces should be localized along the

reduced dimensional materials in the out-of-plane direction. This leads to the possibility of

investigating and manipulating localized CT excitons in the 2D landscape of the respective MDH.

Moreover, owing to the differences in the density of states and dielectric screening environments

on either side of the heterostructure, the mechanism of the CT exciton formation and the

consequent parameters that can control it may be fundamentally different compared to all-2D

systems22. Therefore, MDHs present a new platform to investigate charge-transfer physics and

subsequent exciton formation in MDHs, and will have a broader technological impact on many

device applications23,24.

In this work, we report on the observation of CT excitons in MDHs composed of 2D

transition metal dichalcogenides (TMDCs) and colloidal semiconducting CdSe/CdxSZn1-xS

core/shell nanoplates (NPLs). Even though these nanoplates are colloidal semiconducting

nanocrystals, their density of states more resemble a step-like quasicontinuum similar to a 2D

electronic system25. Therefore, these nanoplates are known as quasi-2D (Q2D) systems. We adopt

tip- enhanced photoluminescence (TEPL) nano-spectroscopy to resolve the spectral signature of

CT excitons from a single NPL/2D heterointerface. Taking advantage of large tunability of the

band structure as a function of shell thickness of CdSe/CdxSZn1-xS based core/shell NPLs and

combining them with monolayer MoSe2 and WSe2 we are able to tune the CT exciton up to 120

meV. Our work presents primary experimental evidence of the presence of CT excitons with large

tunability in a MDH system.

Figure 1. Micro- and nano-optical characterization of CT excitons in MDHs. (a) Schematic

representation of TEPL measurements of the MDHs containing TMDC monolayers (MoSe2 and WSe2)

on top of CdSe/CdSxZn1-xS core/shell NPLs on an Au (or Al2O3/Au) substrate. For the electric field

dependent study an out-of-plane bias was applied through the tip and metal substrate. (b) Optical image

of one representative MDH device investigated in this work. Area of interest (AOI) is outlined by a

dashed rectangle. (c), (d) Far-field PL intensity map of NPL and MoSe2 acquired for the AOI region. (e)

PL intensity overlay image using (c) and (d) showing the MDH interface formation on a MoSe2

monolayer. Scale bar is 10 µm. (f) two representative far-field PL spectra of MoSe2 and MDH acquired

from two nearby pixels as marked in the overlay image. NPL Spectral regions are multiplied by 5 for

better visibility. Orange shades are the spectral regions for which PL maps were created for NPL and

MoSe2 respectively. (g), (h) AFM topography and corresponding TEPL intensity map of NPLs on

monolayer MoSe2 acquired simultaneously. (i) Two representative TEPL spectra averaged over the

rectangle areas marked by 1 and 2 in the TEPL image.

Results and Discussions

Fig. 1a presents a schematic of the TEPL configuration used to characterize MDHs in this study.

The MDHs containing monolayer MoSe2 (or monolayer WSe2) and CdSe/CdxSZn1-xS core/shell

NPLs have three different excitons: two in-plane excitons from the TMDCs and NPL respectively

and one out-of-plane CT exciton across the MDH interface as schematically presented. A gold tip

was used to excite the plasmonic field underneath using 633 nm excitation. Fig. 1b shows an

optical image of one of the representative MDH samples studied in this work. Details of the MDH

device fabrication, NPLs synthesis and characterization can be found in the method section and

the supplementary information section I. Far-field PL intensity maps created for NPLs and MoSe2

and their overlay image for the area of interest (AOI) region marked in the optical image (Fig. 1b)

are shown in Fig. 1c-e respectively. The representative far-field-PL spectra for both MDH and

monolayer MoSe2 are displayed in Fig. 1f. The orange shades are the spectral region for which

the NPL and MoSe2 PL maps were created in Fig. 1c, d. As can be seen in Fig. 1e, the MDHs form

at multiple locations between NPLs and MoSe2. Wherever they form an electronic contact, MDHs

emit CT excitons as revealed by TEPL. However, it is challenging to resolve CT excitons in the

far-field-PL configuration due to the close proximity of this peak to the A exciton of MoSe2 and

the large probing cross-section of the far-field PL geometry (~ 0.2 µm2) compared to a very small

CT exciton emitting area (limited by the spatial extent of NPLs: 6 x 10-4 µm2). These factors

ultimately, lead to very weak CT exciton signals in the far-field PL spectroscopy geometry, (see

supplementary information section II for more details).

The situation can be changed by introducing TEPL, which excites/emits signal locally

under the tip apex with high spatial resolution. Fig. 1g,h show atomic force microscope (AFM)

and corresponding TEPL intensity images of NPLs on MoSe2 respectively. Our sub-20 nm spatial

resolution was enough to resolve CT exciton from a single NPL/2D MDH interface (see

supplementary information Fig. SI-2ii). Despite the excellent sensitivity of TEPL (both

enhancement and spatial resolution), the large extent of the 2D plane can still introduce an

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1TunableLocalizedChargeTransferExcitonsinaMixedDimensionalvanderWaalsHeterostructureMahfujurRahaman1,EmanueleMarino2,3,AlanG.Joly4,SeungukSong1,ZhiqiaoJiang2,5,BrianT.O’Callahan4,DanielJ.Rosen5,KiyoungJo1,GwangwooKim,1PatrickZ.El-Khoury4,ChristopherB.Murray2,5,andDeepJariwala11DepartmentofElectrical...

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