Properties of quantum emitters in different hBN sample types particularly suited for nanophotonic integration

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Properties of quantum emitters in different hBN

sample types particularly suited for nanophotonic

integration

Ambika Shorny1, Hardy Schauffert1, James C.Stewart2, Sajid Ali3, Stefan Walser1,5,

Helmut H ¨

orner1, Adarsh S. Prasad1, Vitaly Babenko2, Ye Fan2, Dominik Eder4, Kristian S.

Thygesen3, Stephan Hofmann2, Bernhard C. Bayer4, and Sarah M. Skoff1,*

1Atominstitut, Technische Universit¨

at Wien, Stadionallee 2, Vienna, 1020, Austria

2Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK

3Computational Atomic-scale Materials Design (CAMD), Department of Physics, Technical University of Denmark,

Fysikvej, 307, Kongens Lyngby, DK-2800, Denmark

4Institute of Materials Chemistry, Technische Universit¨

at Wien, Getreidemarkt 9/165, Vienna, 1060, Austria

5Institut f¨

ur Experimentalphysik, Universit¨

at Innsbruck, Technikerstrasse 25/4, A-6020 Innsbruck, Austria

*sarah.skoff@tuwien.ac.at

ABSTRACT

Single photon emitters in two-dimensional (2D) hexagonal boron nitride (hBN) are promising solid-state quantum emitters for

photonic applications and quantum networks. Despite their favorable properties, much is still unknown about their characteristics

and their atomic origin. We focus on two different kinds of hBN samples that particularly lend themselves for integration with

nanophotonic devices, multilayer nanoﬂakes produced by liquid phase exfoliation (LPE) and a layer-engineered sample from

hBN grown by chemical vapour deposition (CVD). We investigate their inherent defects and ﬁt their emission properties to

computationally simulated optical properties of likely carbon-related defects. Thereby we compare and elucidate the properties

in different sample types particularly suited for photonic quantum networks and narrow down the origin of emitters found in

these samples. Our work is thus an important step towards harnessing the full potential of single photon emitters in hBN.

1 Introduction

Quantum emitters in two-dimensional (2D) materials are attracting much interest due to their remarkable properties

1–6

Particularly, quantum emitters in hexagonal boron nitride (hBN) have been shown to be very stable over a wide temperature

range

7–9

, have bright emission into the zero-phonon line (ZPL) even at room temperature with Debye-Waller (DW) factors

exceeding 0.8

and their transition frequency can be tuned via the Stark effect

. Some quantum emitters in hBN may even

exhibit lifetime-limited emission at room temperature

11,12

, which has not been seen in any other solid-state quantum emitter so

far. In addition, the 2D geometry of the host material particularly lends itself for the integration with nanophotonic devices. For

photonic quantum technologies, such as single photon sources, the bright quantum emitters in hBN around transition energies

of 2.0 eV are particularly suited but so far much remains unknown about the atomic origin and properties of these single photon

emitters

. To fully exploit all of the advantages of solid-state emitters as single photon sources or constituents of photonic

quantum networks and implement scalable devices, near-ﬁeld coupling of the emission to waveguides or microcavities is key.

Here, we aim to shed more light on these quantum emitters by comparing the experimental emission properties of sample

types that are particularly suited for integration with nanophotonics. On one hand side we have chosen layer-engineered hBN

ﬁlms grown by chemical vapour deposition (CVD) and on the other hand commercially available small hBN ﬂakes produced by

liquid-phase exfoliation (LPE).

The CVD grown hBN ﬁlm sample is designed to work for planar waveguide chips and photonic circuits. Here the focus lies

on obtaining high quality extremely thin hBN ﬁlms, more details on the production can be found in

. Such a layer-engineering

approach allows for more control over the position of the emitters and as the thickness of the sample is only three atomic layers,

light scattering by the host crystal is negligible.

The other type of sample that was investigated are LPE hBN nanoﬂakes in liquid suspension that can be commercially

bought and have not been post-treated. Such tiny ﬂakes are an ideal sample for integration with free-standing nanophotonic

waveguides, such as optical nanoﬁbers

, that are naturally integrated with an optical ﬁber network, as they are easily deposited

on such structures. For both hBN samples we study the properties of their most abundant single photon emitters and we verify

arXiv:2210.11099v3 [quant-ph] 19 Feb 2025

Figure 1. (a) Sketch of the layer-engineered CVD grown hBN sample. An optically active hBN layer is sandwiched between

two hBN protection layers. (b) Sketch of the confocal microscope set-up for investigating qantum emitters in hBN.

that their photoluminescence (PL) signatures correspond to the most abundant type found in the respective substrates also

in previous literature

1,16–21

. The investigated defects also occur inherently in these types of samples even without any post-

processing and thus we investigate their characteristics and narrow down their possible origin by comparing computationally

simulated PL properties of the most likely defect types with our experimental data from the two samples.

2 Results and Discussion

2.1 Layer-engineered hBN

For integration with chip-based photonic structures, a thin, ﬂat material is advantageous. For this purpose a stack of three CVD

hBN layers is engineered

, where only the central layer hosts single photon emitters and the two outer layers function as a

protection against bleaching of the emitters (Fig. 1a). This is achieved by transferring three CVD grown monolayers, that have

been cleaned of polymer residue by annealing in air at 450° for about 20 min, on top of each other. Only the central layer hosts

quantum emitters, which have been re-introduced by Ar annealing at about 850°C for 30 min. It has to be noted that these

emitters that were re-introduced via Ar annealing were the same in nature as those that were inherently found beforehand. More

details on the sample preparation can be found in the Methods section and in Stewart et al.14.

We study this trilayer sample using a homebuilt confocal microscope (Fig. 1with a diode laser at 532 nm. The microscope

can either function in confocal or wideﬁeld mode, an example of which can be seen in Fig. 2.

Figure 2. Wideﬁeld image of a trilayer sample of hBN (left) and corresponding confocal image (right) of the marked area

In both cases we collect the ﬂuorescence of the sample and hence regions with quantum emitters appear bright. The

collected light is then sent to a Hanbury-Brown-Twiss (HBT) set-up to check for single photon emission or to a spectrometer to

evaluate the PL proﬁle. Figure 3shows a second-order intensity correlation measurement of an emitter in layer-engineered

hBN showing antibunching for a time delay of

τ=0

and thus proving that it is indeed a single photon emitter. Investigating

these correlations over longer times shows that the emitter we are looking at has at least one metastable energy level. More

information on these measurements and detailed information on the ﬁt is given in the Supplemental Information.

2.2 hBN quantum emitters in liquid phase exfoliated nanoﬂakes

The other type of hBN sample investigated for typical quantum emitters that emit around 2 eV can be purchased commercially

and is useful for dropcasting on photonic components or interfacing with free-standing waveguides as we demonstrate below.

2/15

Figure 3. (a) Second order intensity correlation measurements and corresponding ﬁt (red line) showing the typical

antibunching dip of a single photon emitter. (b) For longer correlation times, photon bunching due to at least one metastable

state can be observed.

The LPE nanoﬂakes purchased from Graphene Supermarket are suspended in a water:ethanol mixture. To estimate their

lateral size and thickness distributions and conﬁrm their structure Atomic force microscopy (AFM), Raman spectroscopy and

transmission electron microscopy (TEM) in bright-ﬁeld (BF) and dark-ﬁeld (DF) mode and selected area electron diffraction

(SAED) are used. For that purpose a droplet of the solution is dropcast onto a SiO

covered Si wafer or lacy carbon TEM grid.

From AFM height proﬁles (Fig. 4a) of individual ﬂakes, we estimate a mean of 160 nm from the lateral platelet size and a

mean of 28 nm for their height with a thickness:length ratio of the ﬂakes of 0.16 conﬁrming their platelet-type morphology.

Considering that a hBN monolayer has a height of 0.34 nm

the observed ﬂakes are few- to multi-layered hBN. The Raman

spectrum in Fig. 4b conﬁrms that the platelets are hBN via its characteristic Raman signal at

∼

1366 cm

−122

. The BF-TEM

Figure 4. (a) AFM micrograph of LPE hBN platelets drop cast from suspension onto SiO2covered Si wafer. (b) Raman

spectrum corresponding to (a). (c) BF-TEM, (d) SAED and (e) DF-TEM images of a single hBN ﬂake deposited onto a lacey

carbon TEM grid.

3/15

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摘要：

PropertiesofquantumemittersindifferenthBNsampletypesparticularlysuitedfornanophotonicintegrationAmbikaShorny1,HardySchauffert1,JamesC.Stewart2,SajidAli3,StefanWalser1,5,HelmutH¨orner1,AdarshS.Prasad1,VitalyBabenko2,YeFan2,DominikEder4,KristianS.Thygesen3,StephanHofmann2,BernhardC.Bayer4,andSarahM.Sk...

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Properties of quantum emitters in different hBN sample types particularly suited for nanophotonic integration

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