nanoscale mri – the quest for a molecular structure …...nanoscale magnetic resonance detection...

Nanoscale MRI – the Quest for a

Molecular Structure Microscope

Support:

IBM

NSF

DARPA QuASAR

John MaminIBM Research Division

Almaden Research Center

Collaborators

Mark Sherwood, Charlie Rettner and Dan Rugar– IBM

Moonhee Kim - IBM

Kenichi Ohno – UC Santa Barbara

David Awschalom – Univ. Chicago

“What you should do in order for us to make more rapid progress is to make the electron microscope 100 times better.” …It is very easy to answer many of these fundamental biological questions; you just look at the thing! You will see the order of bases in the chain; you will see the structure of the microsome. Unfortunately, the present microscope sees at a scale which is just a bit too crude…

http://www.its.caltech.edu/~feynman/plenty.html

It All Goes Back to Feynman

December 1959

There’s Plenty of Room at the Bottom

Xenon atoms on Ni(110) - Eigler and Schweizer (1990)

Quantum corral

Fe on Cu(111) - Crommie et al. 1993

Molecular assembly

Cs8I8 - Hopkinson et al. (1994)

Scanning TunnelingMicroscopy (STM)

Atomic imaging and manipulation on surfaces

4

Nanoscale Magnetic Resonance Detection and Imaging

NanoMRI: Can MRI be extended to do 3D molecular structure imaging?

A protein

complex

mm resolutionnm resolution

• What transducer is both sensitive enough and small enough?

Detection Challenge of Nanoscale NMR

3 nT at 10 nmproton

B ∼

• Two methods: force detection and nitrogen-vacancy centers

• IBM Mamin et al.

A Few Basics of Nuclear Magnetic ResonanceClassical picture of a spin in a magnetic field

0ˆB z

• A spin in a magnetic field experiences a torque, leading

to precession about the field

0 02 /L

B Bω γ µ= = �

γ

• The precession frequency is proportional to the applied field and given by

where is the “gyromagnetic ratio”.

µ

Electron (g=2) 28 GHz/Tesla

1H nucleus 42.6 MHz/Tesla

19F nucleus 40.1 MHz/Tesla

31P nucleus 17.2 MHz/Tesla

13C nucleus 10.7 MHz/Tesla

12C nucleus Not magnetic

Spin type / 2γ π

d

dtγ= ×

0

µµ B

• An rf field at the Larmor frequency can be

used to control spin orientation (ie.,

precession angle).

rf field


Field gradient approach to 3D molecular structure imaging

Resonant slice

Δz = ΔB/G

Magnetic tip to

create field gradient G

• Spatial resolution given by Δz = ΔB/G, where ΔB is the magnetic resonance linewidth

Molecule

Bω γ=


Interferometerlaser beam

Resonant slice(B = 2.70 T)

Magnetic tip

Microwire generating 115 MHz magnetic field

Ultrasensitive cantilever

1H Nuclear spin(γ = 42.6 MHz / Tesla)

irf N

S

18~ 10 N−

Lateral magnetic force

• IBM

Millikelvin MRFM • MRFM cooled by dilution refrigerator • Nanowatt fiberoptic interferometer• Superconducting resonator to generate microwave field• Spring-based vibration isolation

•Tobacco Mosaic Viruses on the end of a MRFM cantilever

•3D map of hydrogen nuclear spin density in virus particles

•100 nm thick shaft

•1 µm thick•mass loading

•Nanomechanical “diving board” sensor for ultrasensitive detection of magnetic forces

•Magnetic Resonance Force Microscopy

• Based on ultrasensitive detection of magnetic force

• Improves MRI sensitivity and volume resolution by factor of 100 million

• 3D imaging with 5 - 10 nm resolution

• Degen, Poggio et al., Proc. Natl. Acad.

Sci. 106, 1313 (2009)

Resonant slice

(B = ω/γ)

Ultrasensitive

cantilever

1H Nuclear spin

Ben Chui

A Very Different Approach: Nitrogen-vacancy centers in diamond

ltdenny.com

NV center: an atomic-size quantum sensor

Optical readout / reset

Spin state with long coherence times

Room temperature operation

Chemically and optically stable

Fluorescence image of individual NV centers

4 µm



NV center: an atomic-size quantum sensor

Optical readout / reset

Spin state with long coherence times

Room temperature operation

Chemically and optically stable


4 µm

Energy levels for optical detection

0s

m =

1sm = ±

2 /g B hν µ∆ =

g

e



4 µm

Energy levels for optical detection

0s

m =

1sm = ±

2 /g B hν µ∆ =

g

e

2.7 2.8 2.9 3.016k

20k

24k

28k

Frequency (GHz)

Co

un

ts/s

2 /g B hν µ∆ =

NV spin echo measurements

τ/2 τ/2

(π/2)x (π/2)-xπy

Laser

Microwaves

NV Polarize NV Readout

min

2

BT

η∼

0 200 400 600 800 10000.6

0.7

0.8

0.9

1.0

SNR =1 at Bac = 9.5 nT (80 sec averaging)

Field amplitude (nT)

Norm

aliz

ed f

luore

scence

Spin echo vs. applied field

With T2 = 1.8 ms (Balasubramanian et al, 2009)

1 sec averaging: SNR = 1 @ 4.3 nTB(t) = Bac sin(ωt)


NV magnetometry measurements

( )zB t dtφ γ= ∫

0 ( )1

0 12

ie

φ−+( )1

0 12

+( ) ( )

( ) ( )

2

2

0 cos / 2

1 sin / 2

P

P

φ

φ

=

=


Detecting 1H NMR in PMMA polymer film

Laser

Ohno et al., Appl. Phys. Lett. (2012).

Mamin et al, Science (2012).

0 500 1000 1500

0.0

0.5

1.0

T2 = 600 µs

Total echo time τ (µs)

Norm

aliz

ed s

pin

echo ξ 0

(τ)

min

2

BT

η∼

NMR detection protocols

No applied NMR rf field

Use selective coupling/decoupling

sequence

Passive detection of proton precession

May be only option if NV T2 is shortMultipulseSequence(CPMG or XY)

Transverse (Larmor) detection

Field from nuclear spins

π/2 π π/2T. Staudacher, et. al., Science (2013)

Longitudinal detection (double resonance)

Long nuclear T1 correlation time optimizes detection if NV has long T2Allows sophisticated NMR measurement (e.g., Fourier transform NMR spectroscopy)Care needed to avoid spurious effect on NV spin echo from NMR pulses

ππ/2 π/2

NMR π pulses

NV spin echo

Field from 1H flips

H. J. Mamin, et al.,Science (2013)

( )zB t dtφ γ∆ = ∫

RF fieldNMR pulsesπ

MW pulsesNV spin echo

( )/ 2y

π± xπ ( )/ 2

yπ

eτ eτ

NMRf NMRf

2.8 3.0 3.2 3.4 3.6

76.9 mT

74.2 mT

NV

spin

echo r

esponse B0 = 79.5 mT

2.8 3.0 3.2 3.63.4

NMRf (MHz)

70 75 80

3.0

3.1

3.2

3.3

3.4

3.5

NM

R d

ip fre

qu

en

cy (

MH

z)

0B (mT)

142.6 MHz T−⋅

Detecting 1H NMR using double resonance

Total echo time = 300 µs

Spin echo vs. RF frequencyDip frequency vs. fieldSlope matches gyromagnetic ratio for protons

Mamin et al, Science (2013)

A “simple” NMR imaging experiment

532 nm excitationFluorescence readout

Diamond substrate

12C

layerNV center

10 nm

microwire

Applied fieldB0 AFM cantilever

PMMA particle

155 µm

• Key objective: Discover and resolve issues related to scanning organic samples over a near-surface NV center

• IBM

•Diamond

•1 µm

•PMMA

•sample

•NV center

•Why not so simple?

•Challenges:

• Near-surface spin decoherence

• NV photo-stability

• Ubiquitous hydrogen layer

• Surface roughness

• SNR!!

• Long scanning times

•10 nm

•1H contamination

•layer

• IBM

•Tuning-fork AFM for NMR Imaging

•attocubes

•magnet

• Tuning-fork sensor eliminates need for AFM optics

• No spurious illumination of the NV center

•temperature sensor

•Tuning-fork AFM for NMR Imaging

•Tuning fork

•Glass probe with polymer

sample

•Gold

microwires on

diamond

•PMMA

•5 µm

•200 µm

5 µm

Interference fringes from cantilever

Nanotip contact point

microwire

50 µm

Cantilever with PMMA nanotip sample

Sample for 1D NMR imaging experiment

Reflection image of nanotip in contact with diamond surface

240 260 280 300 320 340 360

0.2

0.4

0.6

0.8

1.0

NV

co

he

ren

ce

τ (ns)

Sample in contactXY8-96

pulse sequence

NMR detection protocol for imaging

•Multipulse sequence for transverse (Larmor) detection

Find NMR signal vs.

position using 3-point dip

measurement

T. Staudacher, et. al., Science (2013)

•Dip in echo response when τ = 1/2fn

fn = 1.64 MHz

for B0 = 386 Gauss

• IBM

Proton NMR signal vs x position

• Selected Line Scans

• Spatial resolution ~ 12 nm

Lateral position (100 nm per division)

NM

R s

ign

al, s

(x,y

)

(0.2

5 p

er

div

isio

n)

12 nm

2D NMR image

300nm

Rugar et al., Nature Nano. (2015)

Another 2D NMR image

200 nm

(200 nT)2

(New sample and different NV)

Related work: Haberle et al, Nat Nano (2015); DiVience et al, Nat Nano (2015)

Rugar et al., Nature Nano. (2015)

Background Signal From Adsorbate Layer Present

XY8-96MultipulseSequence

Use multipulse sequence for transverse (Larmor) detection

Field from nuclear spins

π/2 π π/2

fn = 1.645 MHz

for B0 = 386 Gauss

240 260 280 300 320 340 360

0.2

0.4

0.6

0.8

1.0

NV

co

he

ren

ce

τ (ns)

Sample retracted

Sample in contactXY8-96

pulse sequence

τ

Signal from adsorbate layer

-10 -5 0 5 100

1

2

3

4

5

6

Lateral position

Vert

ical positio

n

Point spread functionExpected spatial distribution of NMR signal

• Most of the signal originates within a few nanometers of the surface

• Lateral resolution roughly equal to the NV depth

Cross section of PSF for 10 nm deep NV assuming (111) orientation

Lateral position (nm)

Ve

rtic

al p

ositio

n (

nm

)

2 61/rms

B d∝

Signal per 1H spin

for unpolarized

ensemble

• IBM

T2 vs. depth of NV

What causes decoherence of near-surface NV centers?

NV

Unpaired electron spins from surface

dangling bonds

Data from Jayich group UCSB

• “... reducing the dark-spin density will improve NV based sensing …”

Conventional picture:

Common assumption:

T2 increased 4.6x!

� Surface noise source reduced by 80%

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

Air

T2 = 7 µs

D-Glycerol

T2 = 33 µs

Echo

sig

nal

Echo time (µs)

A Big Surprise with Glycerol on diamond:

Major improvement of near-surface spin coherence

But, why? … Did we somehow reduce the unpaired spins on the surface?

Double electron-electron resonance (DEER) measurement of surface spins

1.00 1.05 1.10 1.150.0

0.2

0.4

0.6

0.8

1.0

1.2

Norm

aliz

ed

DE

ER

sig

na

l

Frequency (GHz)

Air (before D-Glycerol)

NV17

1.00 1.05 1.10 1.150.0

0.2

0.4

0.6

0.8

1.0

1.2

Norm

aliz

ed

DE

ER

sig

na

l

Frequency (GHz)

With D-Glycerol

“Dark spin” inversion pulse

1.0 – 1.2 GHz

NV spin echo pulses

1.78 GHz

Laser

NV initialize NV read

π/2 π π/2

NV

Surface spins

No evidence that glycerol substantially changes surface spin density!

How glycerol reduces near-surface spin decoherence

2 2

z B z z z zH DS g B S d E Sµ= + +

�

Simplified Hamiltonian:

/z

f d E h∆ = ∆�

3.5 mHz m/V 17 MV/m= ×

Surface charge effect:

NV center5 nm

e−

2

0

1 2 17 MV/m

4 1d

eE

dπ ε κ∆ = =

+

Δf = 60 kHz

/z

f d E h∆ = ∆�

mHz3.5

V/m

Diamond in air with d = 5nm

3.5 mHz m/V 2.4 MV/m= ×

2

0 Gly

1 2 2.4 MV/m

4d

eE

dπ ε κ κ∆ = =

+

Δf = 8 kHz

/z

f d E h∆ = ∆�

Diamond in glycerol

Gly 42κ =

0 10 20 30 40

0.0

0.2

0.4

0.6

0.8

1.0

Ech

o s

ign

al

Echo time (µs)

Propylene carbonate

T2 = 12 µs

Air

T2 = 5 µs

0 20 40 60 80 100

0.0

0.2

0.4

0.6

0.8

1.0

Air

T2 = 7 µs

D-Glycerol

T2 = 33 µs

Echo

sig

na

l

Echo time (µs)

Near-surface T2 with three different liquids

0 10 20 30 40

0.0

0.2

0.4

0.6

0.8

1.0

Air

T2 = 12.7 µs

Oil

T2 = 11.7 µs

Ech

o s

ign

al

Echo time (µs)

NV17

D-GlycerolPropylene carbonate

Immersion oil

4.6x improvement 2.1x improvement No improvement

Dielectric constants: 43Glyκ = 64

PCκ = 2.3

Oilκ =

• High dielectric constant gives improved spin coherence

• Suppresses electric field noise due to fluctuating surface charge

• Kim et al, PRL (2015)

Summary

• Magnetic resonance imaging can be pushed into the nanoscale

– 5-10 nm resolution in 3D with MRFM (low temperature)

– 12 nm resolution in 2D with NV centers (room temperature)

– Single nuclear spin detection within diamond lattice

– SNR is key!

• Understanding and mitigating surface-induced decoherence is key

– Electric field effects are significant

• Much work is still to be done

– NVs closer to the surface

– Implementation of large field gradient to allow 3D imaging

– More interesting samples!


Acknowledgments

• IBM colleagues

– Dan Rugar

– Mark Sherwood

– Moonhee Kim

– Charlie Rettner

– Noel Arellano

– Kumar Virwani

– Jane Frommer

• Collaborators, past and

present

– Ben Chui

– Christian Degen

– Martino Poggio

– David Awschalom

– Kenichi Ohno

• Funding: IBM, DARPA, ARO and NSF

Dan Moonhee Mark

Rugar Kim Sherwood

Christian Degen

Martino Poggio

nanoscale mri – the quest for a molecular structure …...nanoscale magnetic resonance detection...

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