Icon is not displaying in push notifications in ionic

I am trying to display app icon in notification. But it is displaying blank icon.

I have given pushoptions as below:

const options: PushOptions = {
android: {
titleKey: ‘App’,
sound: ‘true’,
icon: ‘res/drawable/notification_icon’,
topics: [‘MyTopic’]
ios: {
alert: ‘true’,
badge: false,
sound: ‘true’
windows: {}

and copied the icon image as below which has resolution of 40*40px.

<resource-file src="resources/notification_icon.png" target="res/drawable/notification_icon.png/>

Is there anything that I am missing? Iam getting the notification as below: INUQL