Improving the NV generation efficiency
by electron irradiation
Bowen Zhao (赵博文)
1,2
, Yang Dong (董 杨)
1,2,
*, Shaochun Zhang (张少春)
1,2
,
Xiangdong Chen (陈向东)
1,2
,WeiZhu(祝 巍)
3
, and Fangwen Sun (孙方稳)
1,2
1
CAS Key Laboratory of Quantum Information, University of Science and Technology of China,
Hefei 230026, China
2
CAS Center for Excellence in Quantum Information and Quantum Physics,
University of Science and Techn ology of China, Hefei 230026, China
3
Hefei National Laboratory for Physical Science at Microscale, Departm ent of Physics,
University of Science and Techn ology of China, Hefei 230026, China
*Corresponding author: dongy13@ustc .edu.cn
Received April 26, 2020; accepted June 23, 2020; posted online July 15, 2020
The nitrogen vacancy (NV) center in diamond has been well applied in quantum sensing of electromagnetic field
and temperature, where the sensitivity can be enhanced by the number of NV centers. Here, we used electron
beam irradiation to increase the generation rate of NV centers by nearly 22 times. We systematically studied the
optical and electronic properties of the NV center as a function of an electron irradiation dose, where the de-
tection sensitivity of magnetic fields was improved. With such samples with dense NV centers, a sub-pico-Tesla
sensitivity in magnetic fields detection can be achieved with optimal controls and detections.
Keywords: quantum sensing; nitrogen vacancy center generation; electron irradiation.
doi: 10.3788/COL202018.080201.
The nitrogen vacancy (NV) center in diamond has at-
tracted great interests in the scientific community due
to its quantum optical
[1–6]
and spin properties
[7–11]
. Most
applications in quantum sensing tend to use higher con-
centrations of ensemble NV center because of their
stronger fluore scence emission and higher signal-to-noise
ratio
[12–14]
than single NV centers. The measured sensitivity
is then proportional to 1∕
N
p
[15,16]
, where N is the number
of NV centers. In an idealized case in which all other
parameters are held constant, increasing the conversion
efficiency of NV centers in a fixed detection volume will
result in enhanced sensitivity in two ways: first, by in-
creasing the NV center concentration and thus the num-
ber of collected photons from the NV center ensemble; and
second, by decreasing the concentration of the associated
background fluorescence, which improves measurement
contrast.
The conversion efficiency of the NV center in diamond
depends on the presence of nitrogen atoms and vacancies
in the crystal lattice. High-energy particle irradiation,
such as electrons, protons, neutrons, or ions, can create
vacancies in the crystal
[17,18]
. In this process, the high-
energy particle beam interacts with the crystal and has
a certain probability of replacing the carbon atoms in
the crystal lattice, leaving a vacancy. The diamond is then
annealed at 800°C to diffuse the vacancies in the crystal
lattice
[19]
to recombine with the nitrogen atoms already in
the diamond. The final NV center concentration depends
on the concentration of the substituted nitrogen as well as
the irradiation and annealing parameters
[20]
. The upper
limit of NV center production efficiency is limited by
the number of vacancies formed after electron beam
irradiation without increasing the nitrogen content of
the diamond sample
[21]
.
Here, we used electron beam irradiation to enhance the
density of NV centers in diamond. We used a diamond
sample with a nitrogen content of 5 ppm (parts per
million). After different doses of electron beam irradiation
and annealing, the fluorescence saturation curve, spec-
trum, and spin-related properties were measured. The
generation efficiency of the NV center was enhanced by
nearly 22 times, and the sensitivity of the magnetic field
detection was also improved.
The samples used in this study are a chemical vapor
deposition (CVD) grown diamond, which was grown
on type Ib commercial high-pressure high-temperature
(HPHT) (100) oriented single crystal diamond of approxi-
mate dimensions 3 mm × 3 mm × 0.5 mm from Element-6.
Before we used the diamond substrates, they were cleaned
in a mixture of sulfuric and nitric acid (1:2) for 1 h at
200°C and by ultrasonic with deionized water, acetone,
and isopropanol. Prior to growth, the diamond substrates
were pre-treated using a H
2
∕O
2
plasma in order to prepare
their surfaces for single crystal diamond growth. The mi-
crowave plasma CVD system (MPCVD, Seki Technotron
Corp., AX-5250S) was used to grow the samples with
the microwave power of 4.5–5.0 kW at a pressure of
140–160 Torr. The growth temperature was about
1000–1100°C. Hydrogen (5N), 6% CH
4
(5N), and a small
(about 100 ppm in total gas) addition of nitrogen (N
2
)
were used. The growth time for the sample was 48 h,
and the growth rate was about 30 μm/h. After growth,
the sample was separated from the HPHT diamond sub-
strate by laser cutting, and both sides of the growth plates
COL 18(8), 080201(2020) CHINESE OPTICS LETTERS August 2020
1671-7694/2020/080201(5) 080201-1 © 2020 Chinese Optics Letters