Vapor Metalation of Porphyrin by ALD

In-situ QCM analysis of site selective ALD

Conclusions

In-situ GISAXS analysis

In-situ Form Factor Analysis of Site-Selective Atomic Layer Deposition of Metal Oxide Nanoclusters on Vapor Metalated Porphyrin 

Jason R. Avila, Jonathan D. Emery, Omar K. Farha, Michael J. Pellin, Alex B. F. Martinson, and Joseph T. Hupp

Department of Chemistry, Northwestern University and Argonne Northwestern Solar Energy Research Center (ANSER) 2145 Sheridan Road, Evanston, IL, 60208, USA

Using a modular ALD reactor developed at Argonne national laboratory, we can perform high X-ray scattering experiments at Argonne National Laboratory’s Advanced Photon Source to probe the nanoparticle form factor during ALD nucleation and growth in-situ.

UV-Vis confirms the successful vapor metalation of free-base porphyrin Mn using a reactive ALD precursor and a O2/H2O mix to present isolated OH nucleation sites.

In-situ QCM and an island growth model shows MnO nucleation is limited to the OH site on the porphyrin and grows as a hemisphere until its radii exceeds the porphyrins.

In-situ GISAXS shows a significant form factor difference between MnO grown on porphyrin over MnO grown on Si substrates.

Acknowledgements Using an island growth model previously used to quantify the ALD nucleation and growth of metal oxides in SAM films, this study aims to confirm the growth of monodispersed MnOx clusters knowing the fixed radius of the porphyrin and assuming a hemispherical growth geometry.

Model assumes 1) a monondispersed array of nucleation events that will nucleate and grow uniformly and 2) initial growth is limited by a fixed geometry, followed by coalescence to film growth.

Background

The increasing focus on implementing photocatalysts for future sustainable fuel generation has driven many to look toward nature for the ideal system. Examination of enzymatic catalysts indicate the critical property for catalytic activity are nanoscale metallo clusters. Mimicking these cluster systems is synthetically challenging because conventional solution-based methodology can cause aggregation or require capping the cluster, thereby limiting its catalytic active sites. In this work we use site-selective atomic layer deposition (ALD) to grow metal oxide clusters that are spatially isolated and easily controlled. By implementing a tetra-acid free base porphyrin (H2TCPP) nucleation platform we demonstrate the ability to metalate a porphyrin with Mn by ALD precursor exposure to form a spatially isolated hydroxide which acts as a nucleation point for MnO cluster growth. Using in-situ quartz crystal microbalance (QCM) analysis we show through an analytical island growth model that the growth of these clusters is hemispherical with a convergence radii near that expected of the porphyrin platform (0.8 nm). Finally, through in-situ synchrotron GISAXS measurements we find that the structure of MnO grown on porphyrin platforms mirrors the growth behavior determined by QCM measurements. Using this methodology developed in this study we show it is feasible to grow a wide range of well-controlled metallo clusters using the self-limiting nature of ALD.

Abstract

2.5

2.0

1.5

1.0

0.5

0.0

Abs

orba

nce

(a.u

.)

700650600550500

wavelength (nm)

H2TCPP

25 pulse MnEtCp

20 pulse H2O

1 pulse O2/H2O1 cycle MnO

1.4

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Abs

orba

nce

(a.u

.)

700650600550500

wavelength (nm)

ZnTCPP

20 pulse MnEtCp

0.5

0.4

0.3

0.2

0.1

0.0

Abs

orba

nce

(a.u

.)

700650600550500

wavelength (nm)

H2TCPP

2 pulse TMA

Using a wall mounted QCM we can measure the nucleation and growth of MnOx clusters grown by ALD with nanogram resolution

Once metalated, the growth of MnO on the Mn porphyrin shows a hemispherical growth up to 20 cycles before a linear growth rate is observed, similar to what is expected from the island growth model.

Fitting to the model gives a convergence radii similar to the radii of the porphyrin and a linear growth rate similar to that measured by ellipsometry

Vapor metalating with DEZ to form ZnTCPP (which does not have an axial ligand) shows a higher convergence radius indicating the axial OH ligand on the porphyrin is required for growth.

Evolution of thickness (µ) – growth of hemisphere normal to the surface

Radius of convergence (Rcov) – fixed radii before cluster

coalescence.

Nucleation density (Nd) – density of nucleation at defect sites in the

SAM film (pin holes).

thic

knes

s ev

olut

ion/

mas

s ga

in

AB ALD cycles

Rcov

Coalescence

and film gro

wth

Hem

isph

eric

al is

land

gro

wth

Nilsen, O.; et.al. J. Appl. Phys. 2007, 102, 24906Avila, J. R.; et. al. Appl Mater. and Inter.2014, 6, 11891

O2/H2O mix

MnII(CpEt)2 X ALD cycles

Free base porphyrin loaded substrate

ALD clusters of metal oxides/sulfides

Metalated with isolated OH nucleation site

N

N N

N

OH

O

HO

O

O

OH

OH

O

Mn

OH

~1.5 nm diameter Rcov = 0.75 nm Riha, S. C.; et.al. Rev. Sci. Instrum. 2012, 86, 94101

O O

=

Porph

yrin

func

tiona

lized

subs

trate

Passiv

atio

n

of in

ters

titia

l

spac

ed

Met

alat

ion

by A

LD

Model fitted growth

Yanguas-Gil, A.; et.al. Chem. Mater. 2013, 25, 4849-4860

Effect of Interstitial spaces

To establish if the interstitial spaces play a role in the growth morphology of MnO on porphyrins, acetylacetone (AcAc) was vapor deposited to poison any surface hydroxyls before the vapor metalation step.

The Mn metalated case shows no difference in cluster growth with or without AcAc

Once H2TCPP is functionalized on a surface it was then exposed to the Mn(CpEt)2 ALD precursor in order to metalate the porphyrin

UV-Vis shows once exposed to the Mn ALD precursor there is a reduction in the number of Q bands from 4 to 2, and the formation of 2nd soret peak at 475 nm characteristic of a MnIII metalated porphyrin with a ligand normal to the porphyrin ring

UV-Vis was also examined in a N2 environment to determine the mechanism of metalation and ligation using the MnII ALD precursor. Once metalated with MnII water exposure does little to properly oxidize the porphyrin.

O2/H2O mix is needed to oxidize the Mn porphyrin and ligate a hydroxo group for the desired isolated nucleation site

Some degradation of H2TCPP and ZnTCPP porphyrin is observed due to the reactivity of the Mn(CpEt)2 precursor

Can also metalate using diethylzinc (DEZ) and trimethylaluminum but can severely degrade the porphyrin due to high reactivity

UV-Vis under N2

Rochford, J.; et.al. J. Am. Chem. Soc. 2007, 129, 4655-4665

Characterization of MnOx Clusters with Island Growth Model

Porphyrin Nucleation Platform

1 cycle 5 cycle 10 cycle 20 cycle

SiMnO

N

N N

NO

OO

O

O

O

O

O

Mn

OH

Surface Binding Geometry

Using a tetracarboxyphenyl porphyrin with carboxylic acid groups functionalized to the meta positions of the phenyl moiety achieve a binding geometry presents an isolate nucleation sites (-OH) separated by the diameter of the porphyrin ring (~1.5 nm).

Using a solution loaded ZnTCPP shows faster nucleation and film growth than the Mn metalated case indicating the interstitial spaces nucleate quickly.

With AcAc we see a long nucleation delay confirming the interstitial spaces are the primary growth mechanism without an axial ligand on the porphyrin

The difference between the vapor (above) and solution (right) metalated is attributed to increased steric and hydrophobic effects from unreacted precursor ligands

Guinier–Porod fitted radii

Thanks to Jeffery Klug, Matthew Weimer, and Angel Yanguas-Gil for their assistance in setting up and using the in-situ tool.

Scattering for the MnO grown on Mn porphyrin indicates a rougher surface over the bare Si control substrate, with scattering at the high scattering angle indicating particle formation.

MnO grown on Si shows an increase in scattering intensity with minimal comparable roughness

In-plane cuts (qy) shows the peak formation for the MnTCPP indicating discrete particle formation with increasing ALD cycles.

MnO grown on Si does not observe formation of a peak, only observing an increase in scattering intensity with increasing MnO deposition.

Taking out-of-plane cuts (qz) near the q = 0.2 of the scattering signal allows to approximate a particle radii using Guinier-Porod fit.

Assuming a truncated sphere, fitting for radii shows two growth regimes MnO grown on porphyrin. A shallow increase in radii from 1-5 cycles, followed by a second growth regime that matches the growth of MnO on Si.

Further examination as to the discrepancy between the MnO cluster development measured by QCM and GISAXS are ongoing.

Metalation effect on ZnTCPP Metelation with TMA

SiMnO