Optical and Spin Properties of NV Center Ensembles in Diamond Nano-Pillars

At a Glance

Metadata	Details
Publication Date	2022-04-29
Journal	Nanomaterials
Authors	Kseniia Volkova, Julia Heupel, Sergei Trofimov, Fridtjof Betz, Rémi Colom
Institutions	Helmholtz-Zentrum Berlin für Materialien und Energie, University of Kassel
Citations	21
Analysis	Full AI Review Included

Executive Summary

This research details the successful fabrication and characterization of nitrogen-vacancy (NV) center ensembles integrated into diamond nano-pillars, targeting enhanced quantum sensing capabilities for engineering applications.

Material and Fabrication: Nano-pillars (up to 1 µm diameter) were fabricated in nitrogen-rich Type Ib diamond ([100] and [111] orientations) using electron beam lithography (EBL) and inductively coupled plasma reactive ion etching (ICP-RIE).
NV Center Creation: NV ensembles were created via 6 keV He⁺ ion bombardment (8 x 10¹³ ions/cm² dose) followed by 1000 °C annealing.
Optical Enhancement: Finite element analysis and measurements confirm significant fluorescence enhancement (up to 8x) into a high-NA objective (0.95), peaking at approximately 200 nm pillar diameter, due to improved photon collection efficiency.
Ensemble Density: Estimated NV counts per pillar are high: 4300 ± 300 NVs ([100]) and 520 ± 120 NVs ([111]). The lower count in [111] is attributed to reduced vacancy creation efficiency during ion bombardment.
Spin Coherence: Measured electron spin coherence times (T₂) were 420 ns ([100]) and 560 ns ([111]), typical for NV centers in high-nitrogen concentration diamond.
Sensing Performance: Calculated maximum achievable magnetic field sensitivity for the [100] pillars is 11.6 nT/√Hz, suitable for wide-field and scanning probe applications measuring relatively strong fields.
Lifetime Characteristics: Fluorescence decay is bi-exponential (τ₁ ~2 ns, τ₂ ~8 ns), significantly shorter than single NV centers in bulk diamond, likely due to coupling and interaction with bombardment-induced defects (P1 centers).

Technical Specifications

Parameter	Value	Unit	Context
Diamond Type	Ib (HPHT)	N/A	Nitrogen-rich starting material
P1 Center Concentration	65 to 68	ppm	Substitutional nitrogen concentration
Crystal Orientation Tested	[100] and [111]	N/A	Substrate orientations
Ion Implantation Energy	6	keV	He⁺ ion bombardment for vacancy creation
Ion Implantation Dose	8 x 10¹³	ions/cm²	Total dose applied
Annealing Temperature	1000	°C	Vacuum annealing for NV formation
Annealing Pressure	<10^-7	mbar	Ultra-high vacuum
Estimated NV Count ([100])	4300 ± 300	NVs/pillar	Average for ensemble pillars
Estimated NV Count ([111])	520 ± 120	NVs/pillar	Average for ensemble pillars
Pillar Height ([100] sample)	2.2 ± 0.2	µm	Longer etch time (11 min)
Pillar Height ([111] sample)	1.4 ± 0.2	µm	Shorter etch time (8 min)
Etching Rate ([100])	180-220	nm/min	Determined from SEM measurements
Etching Rate ([111])	150-200	nm/min	Determined from SEM measurements
Fluorescence Enhancement	Up to 8	x	Compared to flat interface (NA=0.95)
Optimal Pillar Diameter	~200	nm	For maximum fluorescence enhancement
ODMR Contrast (Maximum)	~5	%	Optically detected magnetic resonance
Coherence Time T₂ ([100])	420 ± 40	ns	Electron spin coherence time
Coherence Time T₂ ([111])	560 ± 50	ns	Electron spin coherence time
Relaxation Time T₁ ([100])	162 ± 11	µs	Electron spin relaxation time
Relaxation Time T₁ ([111])	174 ± 24	µs	Electron spin relaxation time
Calculated Sensitivity ([100])	11.6	nT/√Hz	Maximum achievable magnetic field sensitivity
Calculated Sensitivity ([111])	26.8	nT/√Hz	Maximum achievable magnetic field sensitivity

Key Methodologies

The fabrication process combines ion implantation, annealing, and nanofabrication techniques to create structured NV ensembles:

Material Preparation: Type Ib diamond substrates ([100] and [111] orientation) were cleaned using solvents (acetone, IPA) and piranha acid (3:1 H₂SO₄:H₂O₂).
Vacancy Creation: Samples were bombarded with 6 keV He⁺ ions at a dose of 8 x 10¹³ ions/cm² to create vacancies near the surface (depth profile simulated by SRIM).
NV Formation: Samples were annealed at 1000 °C for two hours in ultra-high vacuum (<10^-7 mbar) to mobilize vacancies, allowing them to combine with substitutional nitrogen (P1 centers) to form NV centers.
EBL Mask Definition:
- A 7 nm Au conductive layer was deposited.
- Positive EBL resist (AR-P 617.06) was spin-coated.
- Pillar arrays (nominal 100 nm and 200 nm circles) were defined using EBL, testing various electron doses (42 µC/cm² to 12.6 mC/cm²) to control proximity effects and final diameter.
- Resist was developed using MIBK:IPA (1:3).
Hard Mask Deposition and Lift-Off:
- A 200 nm Au hard mask (with 5 nm Ti adhesion layer) was deposited.
- Lift-off was performed using DMSO (80 °C overnight) or 96% H₂SO₄ (5 min) to remove the resist and leave only the circular Au/Ti mask features on the diamond surface.
Diamond Etching (ICP-RIE):
- An Oxford Instruments PlasmaLab 100 system was used for dry etching.
- Parameters: 1000 W ICP power, 200 W RF power, 10 sccm O₂ flow, 30 °C substrate temperature, and 5 mTorr (0.7 Pa) chamber pressure.
- Etch Time: 11 min for [100] samples; 8 min for [111] samples.
Mask Removal: The Au mask was removed with potassium iodide solution (KI:I₂ 4:1 mixture), and the Ti adhesion layer was removed with 10% HF solution.
Characterization: Pillars were analyzed using SEM (morphology/size), confocal microscopy (fluorescence mapping), time-correlated photon counting (lifetime), and pulsed/CW ODMR (spin properties T₁, T₂).

Commercial Applications

The development of structured NV ensemble sensors in low-cost, high-nitrogen Type Ib diamond enables several applications, particularly where high photon count rates and moderate sensitivity are required.

Quantum Sensing and Metrology:
- Scanning Probe Magnetometry: The nano-pillars can serve as robust tips for scanning probe systems, allowing nanoscale imaging of magnetic fields (e.g., investigating magnetic layers or hard drives).
- Wide-Field Imaging: NV ensembles provide high photon flux, enabling faster data acquisition and simpler experimental setups (like wide-field microscopes) for magnetic field mapping over larger areas.
Low-Cost Quantum Hardware:
- The use of readily available and cheaper HPHT Type Ib diamond (used widely in industrial grinding/cutting) allows for the low-cost production of structured quantum sensors, bypassing the need for expensive, ultra-pure CVD diamond.
Solid-State Quantum Architectures:
- The pillars act as photonic structures, improving the collection efficiency of emitted photons, which is crucial for integrating NV centers into scalable solid-state quantum technology architectures.

View Original Abstract

Nitrogen-vacancy (NV) color centers in diamond are excellent quantum sensors possessing high sensitivity and nano-scale spatial resolution. Their integration in photonic structures is often desired, since it leads to an increased photon emission and also allows the realization of solid-state quantum technology architectures. Here, we report the fabrication of diamond nano-pillars with diameters up to 1000 nm by electron beam lithography and inductively coupled plasma reactive ion etching in nitrogen-rich diamonds (type Ib) with [100] and [111] crystal orientations. The NV centers were created by keV-He ion bombardment and subsequent annealing, and we estimate an average number of NVs per pillar to be 4300 ± 300 and 520 ± 120 for the [100] and [111] samples, respectively. Lifetime measurements of the NVs’ excited state showed two time constants with average values of τ1 ≈ 2 ns and τ2 ≈ 8 ns, which are shorter as compared to a single color center in a bulk crystal (τ ≈ 10 ns). This is probably due to a coupling between the NVs as well as due to interaction with bombardment-induced defects and substitutional nitrogen (P1 centers). Optically detected magnetic resonance measurements revealed a contrast of about 5% and average coherence and relaxation times of T2 [100] = 420 ± 40 ns, T2 [111] = 560 ± 50 ns, and T1 [100] = 162 ± 11 μs, T1 [111] = 174 ± 24 μs. These pillars could find an application for scanning probe magnetic field imaging.

Tech Support

Original Source

DOI: https://doi.org/10.3390/nano12091516

References

2008 - Nanoscale imaging magnetometry with diamond spins under ambient conditions [Crossref]
2008 - Nanoscale magnetic sensing with an individual electronic spin in diamond [Crossref]
2011 - Electric-field sensing using single diamond spins [Crossref]
2013 - Nanometre-Scale Thermometry in a Living Cell [Crossref]
2013 - High-Precision Nanoscale Temperature Sensing Using Single Defects in Diamond [Crossref]
2014 - Electronic properties and metrology applications of the diamond NV− center under pressure [Crossref]
2010 - Monolithic Diamond Optics for Single Photon Detection [Crossref]
2012 - Coupling of Nitrogen-Vacancy Centers to Photonic Crystal Cavities in Monocrystalline Diamond [Crossref]
2010 - A Diamond Nanowire Single-Photon Source [Crossref]
2015 - Nanoengineered Diamond Waveguide as a Robust Bright Platform for Nanomagnetometry Using Shallow Nitrogen Vacancy Centers [Crossref]